No Content-Length for 204 or 304.

It's better that the field `Content-Length` is cleared
when the status code is 204 or 304, according to RFC 2616
sec 4.4, sec 10.2.5 and sec 10.3.5.
Merge pull request #173 from favonia/no-content-length-for-304

No Content-Length for 204 or 304.