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 )

These extraneous headers don't cause a problem with most browsers, but I ran into a problem with Twilio's ( http://www.twilio.com ) HTTP proxy that is likely to stem from this.

As a side-note, it would be nice to be able to make vserver rules that apply to certain response codes; that way, I could have made a 304 rule that deleted these two headers, and experimented with the fix without changing the source.