Expose the response body as a IO object.

Gives direct access to the underlying IO object of a response body,
which wasn't possible before,
response.body.copy_stream for instance would fail if you have set a
another charset(encoding), which isn't possible with IO objects anyway.

It also wasn't helping that every ruby web server would have to be
rewritten to check if response.body.copy_stream would exist.
Merge branch 'master' into response-as-io

Conflicts:
	lib/webmachine/spec/adapter_lint.rb
Merge pull request #165 from seancribbs/response-as-io

Expose the response body as a IO object.