Don't return "Not Modified" (304) if misc->cacheable has been lowered.
This prevents always sending 304 answers to a browser supplying a
if-modified-since header when the page is expected to expire due to
some dynamic RXML. However, if the expire time isn't lowered below
INITIAL_CACHEABLE (5 min) the browser will still get 304 answers
forever (or until the page really is modified).
Set expire time to 1970 if misc->cacheable is 0. This avoids problem
with immediate expiration and clock skew between server and client for
browsers using HTTP/1.0.