Hyper should skip automatic Content-Length header for HTTP 304 responses

[KamilaBorowska]

It's unnecessary, see RFC 7232.

Currently Hyper for empty bodies with Content-Length header unset automatically sets the value 0 which is incorrect for HTTP 304 responses, the correct values are either to not provide Content-Length (recommended by specification) or to provide Content-Length of a full response.

[seanmonstar]

This gets set here:

hyper/src/proto/h1/role.rs

Line 479 in 8d70bac

extend(dst, b"content-length: 0\r\n");

A check could be added there for cases when there should be no header added.