Export headers in GithubException (#1887)

Since the headers that led to an exception are also useful, firstly pass
them into the constructor, and then export them in a property. Test one
specific use case to make sure of coverage.

Fixes #1814

github/GithubObject.py

@@ -300,7 +300,7 @@ class CompletableGithubObject(GithubObject):
     def __complete(self):
         if self._url.value is None:
             raise GithubException.IncompletableObject(
-                400, "Returned object contains no URL"
+                400, "Returned object contains no URL", None
             )
         headers, data = self._requester.requestJsonAndCheck("GET", self._url.value)
         self._storeAndUseAttributes(headers, data)