Allow GithubObject.update() to be passed headers (#1300)

SourceImport.update() is really GithubObject.update(), which just calls
GET on the object's URL. Refetching a source import requires an
additional header, so change GithubObject.update() to pass any it is
given, and add a test for SourceImport.update()

Fixes #1297

github/GithubObject.py

@@ -279,7 +279,7 @@ class CompletableGithubObject(GithubObject):
         self._storeAndUseAttributes(headers, data)
         self.__completed = True
 
-    def update(self):
+    def update(self, additional_headers=None):
         '''
         Check and update the object with conditional request
         :rtype: Boolean value indicating whether the object is changed
@@ -289,6 +289,8 @@ class CompletableGithubObject(GithubObject):
             conditionalRequestHeader[Consts.REQ_IF_NONE_MATCH] = self.etag
         if self.last_modified is not None:
             conditionalRequestHeader[Consts.REQ_IF_MODIFIED_SINCE] = self.last_modified
+        if additional_headers is not None:
+            conditionalRequestHeader.update(additional_headers)
 
         status, responseHeaders, output = self._requester.requestJson(
             "GET",