Don't have double content-length headers snoyberg/keter#52

Pinging @kazu-yamamoto. Can you review this patch? The issue is that,
currently, any responseFile respone which sets content-length ends up
with two content-length headers. I'm also going to patch wai-app-static
to not provide a content-length header, since it's incorrect in the case
of range requests.
Don't send content-length from wai-app-static snoyberg/keter#52