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/SourceImport.py

@@ -23,6 +23,7 @@
 ################################################################################
 
 from __future__ import absolute_import
+from github import Consts
 import github.GithubObject
 
 
@@ -143,6 +144,10 @@ class SourceImport(github.GithubObject.CompletableGithubObject):
         self._completeIfNotSet(self._vcs_url)
         return self._vcs_url.value
 
+    def update(self):
+        import_header = {"Accept": Consts.mediaTypeImportPreview}
+        return super(SourceImport, self).update(additional_headers=import_header)
+
     def _initAttributes(self):
         self._authors_count = github.GithubObject.NotSet
         self._authors_url = github.GithubObject.NotSet