Unzipped responses should have Content-Length removed

[s25g5d4]

Some server do add Content-Length in response header. For zipped responses, the content-length is the compressed, compact size of data of course. But because in PR #72 zipped responses will get decompressed, if content-length is presented in original response header, it should be updated, or, removed. I'm willing to send a PR to fix it. It is just an one-line fix, I would really appreciate it if you could take a look.

[s25g5d4]

Forgot to say that why I made this ticket is because, I have some SVGs that will be sent gzipped by server. But because the content-length recorded in fixtures are smaller than the actual data size, browser will truncate responses and fail to parse those SVGs.

[marcusschiesser]

@assaf - any plans to release this commit soon?