Fail in Plug.Conn.put_status/2 when the resp had already been sent

Most of the `Plug.Conn` functions raise an `AlreadySentError` when the
response has already been sent. This commit introduces the code and tests that
make this function behave like the others.
Merge pull request #133 from whatyouhide/checks-on-put-status

Fail in Plug.Conn.put_status/2 when the resp had already been sent