Remove content-encoding header when response is decoded

I'm guessing others have different opinions on this, but I believe em-http-request should remove the Content-Encoding header when it decodes a response itself.

This allows em-http-request to operate more consistently with other http libraries (including Net::HTTP) when used in projects like Faraday with interchangeable adapters. It also fits the idea that content encoding should be "transparent" to the application - handled entirely at the protocol level.

As an example, this behavior is currently breaking the Google API Ruby Client (https://github.com/google/google-api-ruby-client/blob/master/lib/google/api_client/gzip.rb) when Faraday uses the em_synchrony adapter since the Google client assumes that if Content-Encoding is set to "gzip", it should decompress it there. Since em-http-request is passing a decompressed body *and* the "Content-Encoding: gzip" header, the Google API Client is failing when attempting to decompress the (already decompressed) body.

Of course, some may be of the opinion that the problem should be fixed higher up (in Faraday or the Google client in this case), but I feel like it's perfectly reasonable for a higher-level project to assume consistency between the Content-Encoding header and body data.

I'd love to hear other thoughts on this.