Make the content-length header more RFC compliant

[Blacksmoke16]

RFC7230 says:

A server MAY send a Content-Length header field in a 304 (Not Modified) response to a conditional GET request (Section 4.1 of [RFC7232]); a server MUST NOT send Content-Length in such a response unless its field-value equals the decimal number of octets that would have been sent in the payload body of a 200 (OK) response to the same request.

and

A server MUST NOT send a Content-Length header field in any response with a status code of 1xx (Informational) or 204 (No Content). A server MUST NOT send a Content-Length header field in any 2xx (Successful) response to a CONNECT request (Section 4.3.6 of [RFC7231]).

This PR updates HTTP::Server to better follow the RFC in regard to when the content-length header is included automatically. Previously it would be set to 0, regardless of the response status, if it was not already set.

This PR does NOT handle the CONNECT request case, this is much less common and can be handled via another PR in the future.

[Blacksmoke16]

Actually after sleeping on this, do we want to go a step further and actually remove the content-length header if its present in these contexts versus just not automatically adding it? I.e. if an application sets it themselves, with a 204 they would still be in violation of the RFC.

[straight-shoota]

Let's leave the scope of this PR as is. That idea could be a follow-up but may need some more discussion first.