added repository collaborator invitation

github/Repository.py

@@ -673,8 +673,13 @@ class Repository(github.GithubObject.CompletableGithubObject):
 
         headers, data = self._requester.requestJsonAndCheck(
             "PUT",
-            self.url + "/collaborators/" + collaborator
+            self.url + "/collaborators/" + collaborator,
+            headers={'Accept': 'application/vnd.github.swamp-thing-preview+json'}
         )
+        # return an invitation object if there's data returned by the API. If data is empty
+        # there's a pending invitation for the given user.
+        return github.Invitation.Invitation(self._requester, headers, data, completed=True) if \
+            data is not None else None
 
     def compare(self, base, head):
         """