Do not request gzipped media responses by default.

For some reason, Twitter's image server will gzip compress the main
profile picture found on a tweet page. Since we don't properly deflate
this gzipped image in _fetch_image_size(), we end up tossing out the
main account's profile image as a possible thumbnail. For some other odd
reason, the profile images of those responding to tweets won't returned
gzipped, meaning we snag some random person's profile image erroneously.

Example: http://redd.it/2gh3mu

Images generally don't benefit from gzip compression. We also chunk
image fetching so it is unlikely we'll grab much more than 1k. Instead
of trying to deflate these oddly gzipped images, I've made
_initialize_request() not fetch gzipped objects by default, and have
modified _fetch_url() to request gzipped objects since it *does*
properly deflate.

:eyeglasses: @umbrae @deimos