PoolManager urlopen() redirect at most once (issue #149)

This pull request fixes the PoolManager urlopen() redirect at most once issue ($149).

Summary of changes:
 1. Add a test case for server redirecting twice (with the dummy server);
 1. Set argument _redirect_ for recursive urlopen() call to the value of the passed in param.

New test passed with Python 2.6 on 32bit Debian 6. Since the fix is just a 1-liner, I think it's safe to assume it works on other platforms.