Fix http.py: Exception -> exception, lint errors, unit test. (#724)

* Exception -> exception.

* Assign exception = None

* Fix lint errors.

* Fix test_media_file_upload_closes_fd_in__del__
diff --git a/tests/test_http.py b/tests/test_http.py
index edaef6d..5aaada6 100644
--- a/tests/test_http.py
+++ b/tests/test_http.py
@@ -213,8 +213,12 @@
   def test_media_file_upload_closes_fd_in___del__(self):
     file_desc = mock.Mock(spec=io.TextIOWrapper)
     opener = mock.mock_open(file_desc)
-    with mock.patch('__builtin__.open', return_value=opener):
-      upload = MediaFileUpload(datafile('test_close'), mimetype='text/plain')
+    if PY3:
+      with mock.patch('builtins.open', return_value=opener):
+        upload = MediaFileUpload(datafile('test_close'), mimetype='text/plain')
+    else:
+      with mock.patch('__builtin__.open', return_value=opener):
+        upload = MediaFileUpload(datafile('test_close'), mimetype='text/plain')     
     self.assertIs(upload.stream(), file_desc)
     del upload
     file_desc.close.assert_called_once_with()