Replace a real HTTP server in test_fetchers with mock urlopen
Don't test redirects in fetchers, it's a urlopen's job.
test_fetchers: Don't test the same behavior many times.
[test_fetchers] Rearranged code as a regular TestCase. +1 to Discoverability!
[test_fetchers] Separate test for URL errors. Also check that disallowed protocols don't even call urlopen.
[test_fetchers] A single test for HTTP errors.
[test_fetchers] Testing user agent.
[test_fetchers] Testing POST data.
[test_fetchers] Removed geturl
[test_fetchers] Removed custom assert functions in favor of calling standard TestCase assert methods.