Adjust Content-Length header when decompressing content

It looks like the `Content-Length` header is incorrect for decompressed responses. I patched it to recalculate the body length and preserve the original header value in the spirit of the current `Content-Encoding` handling.

I'm not positive that the use of `(count (:body resp-c))` is the right way to calculate the new length. Please let me know if there is a better way and I'll update the patch. Thanks!