Do not add Content-Type or Last-Modified headers to 304 responses.

The new behavior is compliant with RFC2616
( http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5 )
Merge branch '304fix' of https://github.com/mrj10/webserver into 304fix
Did some extra cleanups for readibility
Merge branch '304fix'