Handle encoded response content in batch http request in Python 3

[markflyhigh]

Fix issue reported in #249.

The problem happens in Python 3 when execute a http request and encoded content is returned. This breaks email_parser.Parser().parsestr() call in batch.py since the function argument only accepted str (not bytes). There is a callback __ProcessHttpResponse that handles encoded response content but the call happens later.

[coveralls]

Coverage increased (+0.01%) to 91.714% when pulling 28509a8 on markflyhigh:fix-py3-encoding into effc2e5 on google:master.

[markflyhigh]

friendly ping.

[kevinli7]

When I test this out manually in gcloud, this line in batch.py is failing in python2:
mime_response = parser.parsestr(header + content)

Also, I feel like this decoding should have some sort of switch to turn it on and off.

[markflyhigh]

Interesting. The unit tests are passing in Python 2 and 3. Can you give more details how you test with gcloud?

Add a switch to this decoding sounds good to me. I think we can add a response_encoding field to BatchHttpRequest and BatchApiRequest, which also allow people to specify the encoding of the response content.

[markflyhigh]

@kevinli7 PTAL

[kevinli7]

LGTM. @houglum not sure if you guys use batch? Do you want to take a look as well?