DockerClient.secrets is not a property
`DockerClient.secrets` is not decorated with `property` and so must be called rather than just accessed. This is inconsistent with the docs and similar collections on the client attribute (e.g. images, containers, etc.).
diff --git a/docker/client.py b/docker/client.py
index 09abd633..66ef60f3 100644
--- a/docker/client.py
+++ b/docker/client.py
@@ -119,6 +119,7 @@ class DockerClient(object):
         """
         return PluginCollection(client=self)
 
+    @property
     def secrets(self):
         """
         An object for managing secrets on the server. See the
['docker/client.py']