MINOR: http: factor out the content-type checks

MINOR: http: factor out the content-type checks

Let's only look up the content-type header once. This involves
inverting the condition which is not dramatic.

Also, we now always check the value length before comparing it, and we
always reset the ctx.idx before looking a header up. Otherwise that
could make header lookups depend on their on-wire order. It would be
a minor issue however since at worst it would cause some responses not
to be compressed.