Remove "Content-Encoding: gzip" header

When we try to proxy gzipped content, curl (with the current options) returns it uncompressed. We then pass it to the browser/client with the original "Content-Encoding: gzip" header which can confuse the browser (the browser will fail trying to decompress the data because it's not really gzipped).

For example see what happens when you try to proxy "http://gzipwtf.com/" with and without this fix.

The fix prevents this header from being returned to the caller.