fix: do not assume an empty content-type header is JSON (#444)

The parser for HTTP binary made the assumption that if there was no `content-type`
header in the incoming message, it should inject `application/json`. Discussion
about the rationale for this is in #441.

This commit, removes that injection and adds a test to ensure the bytes are
simply not parsed, but just passed along untouched.

Fixes: #441

Signed-off-by: Lance Ball <lball@redhat.com>

Author: lance