Return a response object instead of response.raw

Instead of returning Response.raw, return the Response object, in case we want to do other advanced things with Python Requests, such as those here: http://www.python-requests.org/en/latest/api/#requests.Response

I wanted to take advantage of Response.iter_content() so that I wouldn't have to load an entire file into memory before writing it to the disk. This wasn't possible before, as _do_request() was returning Response.raw.