Return default content type to text/plain. Fixes #89.

This pull request adds our own version of `content-type-response` that does not meddle with response Content-Type unless it determines MIME type by the requests original URI.

This causes the original "text/plain" MIME type default to shine through.

We may still want to change this default to HTML, but I don't know that this is the place to do that (fixing a bug.)

/cc @timewald