Postprocessing: Methods appears in output of get_parameter_as_dict()
**Describe the bug**

The output of `processing.get_parameter_as_dict()` contains methods from oemof.tabular. The methods are e.g. the `build_solph_components` of the oemof.tabular facades. This is not intended - not what one expects as a parameter.

**To Reproduce**
Steps to reproduce the behavior:
1. Create an EnergySystem using oemof.tabular.
2. Call get_parameter_as_dict(es).
3. See the function objects `build_solph_components` in the dictionary.

**Expected behavior**
The function objects should not be part of the dictionary.

**Desktop (please complete the following information):**
 - OS: Xubuntu
 - Python version 3.6

diff --git a/src/oemof/solph/processing.py b/src/oemof/solph/processing.py
index 679a9ae7..0bcfde44 100644
--- a/src/oemof/solph/processing.py
+++ b/src/oemof/solph/processing.py
@@ -267,9 +267,7 @@ def __separate_attrs(system, get_flows=False, exclude_none=True):
             "registry",
             "inputs",
             "outputs",
-            "register",
             "Label",
-            "from_object",
             "input",
             "output",
             "constraint_group",
@@ -277,7 +275,7 @@ def __separate_attrs(system, get_flows=False, exclude_none=True):
         attrs = [
             i
             for i in dir(com)
-            if not (callable(i) or i.startswith(exclusions))
+            if not (callable(getattr(com, i)) or i.startswith(exclusions))
         ]
 
         for a in attrs:
['src/oemof/solph/processing.py']