POST transport: missing return and unnecessary logs

[Desuuuu]

The HTTP POST transport was missing a return in the first error case.

I also removed the two log.Printf calls which seem to have been left there unintentionally.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

Coverage: 78.723% (-0.003%) from 78.725% when pulling 91a2d80 on Desuuuu:post-transport into 622039c on 99designs:master.

[StevenACoffman]

Thanks!