Don't validate for status 204 (no content)

With status 204 (no content),  `MultiJson.decode(full_body)` was failing because `full_body` is empty.

Quick hack: Don't attempt to validate if status is 204.
add test to pass through 204 (no content) response
Merge pull request #59 from ronen/patch-1

Don't validate for status 204 (no content)