`Raxx.set_body` should raise an exception for messages with no body.

[CrowdHailer]

https://tools.ietf.org/html/rfc7230#section-3.3

All 1xx (Informational), 204 (No Content), and 304 (Not Modified)
responses do not include a message body. All other responses do
include a message body, although the body might be of zero length.

Checklist

• test for 100,204,304 responses.
• Add entry to FAQ with link to RFC section
• Add changelog entry

Follow up

The Parser should handle the special case of receiving a 204/304 with a content length and know not to read the body

[CrowdHailer]

Now on master. Will be released in 0.16.0