posse-post-discovery: more aggressively canonicalize urls

remove call to follow_redirects in refetch loop.
converts the following:

* twitter -> https://twitter.com/... (no http and no www)
* facebook -> https://facebook.com/...
* instagram -> http://instagram.com/...
* google+ -> https://plus.google.com/...
posse-post-discovery: clean up canonicalize tests

use for-loops to eliminate redundant assertions for the
same expected result URL
Merge pull request #200 from kylewm/ppd-canonicalize-urls

posse-post-discovery: more aggressively canonicalize urls