set Content-Length correctly

hello,

I had the problem that the `Content-Length` was set incorrectly with ruby 2.
it was set to the size before gzip decompression. I created a fix which always set the actual length of the response body.

I would be happy if you can give me feedback if this fix is okay or if you can reproduce the bug.
or when you have a better solution.

for now I will embed my fork into my gemfile so that the gem works again with the newest ruby version.

thanks!