Do not request gzipped media responses by default.

For some reason, Twitter's image server will sometimes gzip 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. Since this
behaviour only happens sometimes, our scraper eventually finds a
non-gzipped image it can use, meaning we sometimes 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.