Improve handling of HTTP 204 responses.

HTTP 204 responses have no body, but can have any headers they like,
including, counter-intuitively, ones specifying the content type.

This makes sure Boto won't pass a 0 length body to the json parser in
response to a 204 status from Glacier.

Unit test added to catch any regression.
Merge pull request #2726 from twirrim/develop

Improve handling of Glacier HTTP 204 responses. Fixes #2726.