Recalculate content-length

It is needed according to
https://github.com/django/django/blob/master/django/middleware/gzip.py#L46
Added tests to make sure when a response is compressed the 'Content-Length' header is included in the response, otherwise the client (curl for example) will be confused about the content length
Merge pull request #244 from camilonova/patch-2

Recalculate content-length