Fix assertion in add_to_members (#1146)

The role parameter is set to NotSet by default
but the assertion only allowed str and unicode.
So it would always fail. Add NotSet to the assertion
to allow calling the function without a role.

Signed-off-by: Pascal Bach <pascal.bach@nextrem.ch>

github/Organization.py

@@ -311,7 +311,7 @@ class Organization(github.GithubObject.CompletableGithubObject):
         :param role: string
         :rtype: None
         """
-        assert isinstance(role, (str, unicode)), role
+        assert role is github.GithubObject.NotSet or isinstance(role, (str, unicode)), role
         assert isinstance(member, github.NamedUser.NamedUser), member
         put_parameters = {}
         if role is not github.GithubObject.NotSet:
@@ -679,11 +679,11 @@ class Organization(github.GithubObject.CompletableGithubObject):
         :rtype: :class:`github.PaginatedList.PaginatedList` of :class:`github.Project.Project`
         :param state: string
         """
-        
+
         url_parameters = dict()
         if state is not github.GithubObject.NotSet:
             url_parameters["state"] = state
-            
+
         return github.PaginatedList.PaginatedList(
             github.Project.Project,
             self._requester,
@@ -691,7 +691,7 @@ class Organization(github.GithubObject.CompletableGithubObject):
             url_parameters,
             {"Accept": Consts.mediaTypeProjectsPreview}
         )
-        
+
     def get_public_members(self):
         """
         :calls: `GET /orgs/:org/public_members <http://developer.github.com/v3/orgs/members>`_