KHR_lights_punctual contains too many lights
**Describe the bug**
the exporter seems to write too many entries into the "extensions/KHR_lights_punctual/lights" array. In my case it write 10 entries, although its a scene with one point light and nothing else:

```
"extensions" : {
        "KHR_lights_punctual" : {
            "lights" : [
                {
                    "color" : [
                        1,
                        1,
                        1
                    ],
                    "intensity" : 10,
                    "type" : "point",
                    "name" : "Light"
                },
                {
                    "color" : [
                        1,
                        1,
                        1
                    ],
                    "intensity" : 10,
                    "type" : "point",
                    "name" : "Point"
                },
                {
                    "color" : [
                        1,
                        1,
                        1
                    ],
                    "intensity" : 10,
                    "type" : "point",
                    "name" : "Point.001"
                },
                {
                    "color" : [
                        1,
                        1,
                        1
                    ],
                    "intensity" : 100,
                    "type" : "point",
                    "name" : "Light"
                },
                {
                    "color" : [
                        1,
                        0,
                        0.008219769224524498
                    ],
                    "intensity" : 50,
                    "type" : "point",
                    "name" : "Point.001"
                },
                {
                    "color" : [
                        0.007947047241032124,
                        0.007947047241032124,
                        0.007947047241032124
                    ],
                    "intensity" : 100,
                    "type" : "point",
                    "name" : "Light"
                },
                {
                    "color" : [
                        0.028826946392655373,
                        0.028826946392655373,
                        0.028826946392655373
                    ],
                    "intensity" : 10,
                    "type" : "point",
                    "name" : "Point"
                },
                {
                    "color" : [
                        0.010022828355431557,
                        1.5852629431378773e-08,
                        0.0006771537009626627
                    ],
                    "intensity" : 50,
                    "type" : "point",
                    "name" : "Point.001"
                },
                {
                    "color" : [
                        0.004593575838953257,
                        0.005929010454565287,
                        0.009789709001779556
                    ],
                    "intensity" : 50,
                    "type" : "point",
                    "name" : "Point.001"
                },
                {
                    "color" : [
                        0.03310476616024971,
                        3.891099709107948e-08,
                        0.0009548924281261861
                    ],
                    "intensity" : 10,
                    "type" : "point",
                    "name" : "Point.002"
                }
            ]
        }
    },
```

**To Reproduce**
create a new blender scene. delete everything from the scene ('A' + 'Del'). Create a point light source. Export to .gltf with "Punctual Lights" option enabled.
**Expected behavior**
As far as i understand there should be only one light entry in the final .gltf file.

**.blend file/ .gltf**
[alight.zip](https://github.com/KhronosGroup/glTF-Blender-IO/files/3172484/alight.zip)
**Version**
 - OS: Windows 8.1
 - Blender Version 2.80 Beta

diff --git a/addons/io_scene_gltf2/blender/exp/gltf2_blender_gltf2_exporter.py b/addons/io_scene_gltf2/blender/exp/gltf2_blender_gltf2_exporter.py
index 8fca01f6..3eded755 100644
--- a/addons/io_scene_gltf2/blender/exp/gltf2_blender_gltf2_exporter.py
+++ b/addons/io_scene_gltf2/blender/exp/gltf2_blender_gltf2_exporter.py
@@ -229,7 +229,7 @@ class GlTF2Exporter:
         return name + image.file_extension
 
     @classmethod
-    def __get_key_path(cls, d: dict, keypath: List[str], default=[]):
+    def __get_key_path(cls, d: dict, keypath: List[str], default):
         """Create if necessary and get the element at key path from a dict"""
         key = keypath.pop(0)
 
@@ -302,7 +302,7 @@ class GlTF2Exporter:
             # extensions that lie in the root of the glTF.
             # They need to be converted to a reference at place of occurrence
             if isinstance(node, gltf2_io_extensions.ChildOfRootExtension):
-                root_extension_list = self.__get_key_path(self.__gltf.extensions, [node.name] + node.path)
+                root_extension_list = self.__get_key_path(self.__gltf.extensions, [node.name] + node.path, [])
                 idx = self.__append_unique_and_get_index(root_extension_list, extension)
                 return idx
 
['addons/io_scene_gltf2/blender/exp/gltf2_blender_gltf2_exporter.py']