Make http.MediaFileUpload close its file descriptor.

[wangxf123456]

Fixes #575

[tseaver]

Thank you for the patch! In addition to my comment inline, can you please add a testcase in tests/test_http.py which exercises the new code? Something like:

class TestMediaUpload(unittest.TestCase):
...
  def test_media_file_upload_closes_fd_in___del__(self):
    file_desc = mock.Mock(spec=['close'])
    opener = mock.mock_open(file_desc)
    with mock.patch('googleapis.http.open', opener):
      upload = MediaFileUpload(datafile('test_close'))
    self.assertIs(upload.stream(), file_desc)
    del upload
    file_desc.close.assert_called_once_with()

[wangxf123456]

Thanks very much for your review!