fix(server): skip automatic Content-Length header for HTTP 304 responses

[jxs]

closes #1797

[seanmonstar]

This is a great start! This prevents adding a content-length: 0, but I believe there's another case that needs to be address also, which when there was a non-empty body. The body should still be dropped and no content-length should be written, I believe.

[jxs]

updated! dropped the body and not setting the content-length for BodyLength::Known(len). But even if body isn't dropped on match msg.bodyit still is being dropped somewhere else.
Added a new test case to assert body is dropped though

[KamilaBorowska]

HTTP 304 cannot have a body, so the case of HTTP 304 with body shouldn't happen, just saying. From RFC 7232.

A 304 response cannot contain a message-body; it is always terminated by the first empty line after the header fields.

[seanmonstar]

Thanks!