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/GithubException.py

@@ -38,11 +38,12 @@ class GithubException(Exception):
     Some other types of exceptions might be raised by underlying libraries, for example for network-related issues.
     """
 
-    def __init__(self, status, data):
+    def __init__(self, status, data, headers):
         super().__init__()
         self.__status = status
         self.__data = data
-        self.args = [status, data]
+        self.__headers = headers
+        self.args = [status, data, headers]
 
     @property
     def status(self):
@@ -58,6 +59,13 @@ class GithubException(Exception):
         """
         return self.__data
 
+    @property
+    def headers(self):
+        """
+        The headers returned by the Github API
+        """
+        return self.__headers
+
     def __str__(self):
         return "{status} {data}".format(status=self.status, data=json.dumps(self.data))