Update urllib3 to 1.9.1 (latest)

This pull request updates the urllib3 dependency from 1.7.1 to 1.9.1 (the latest).

I had to make a minor change in core.py to get some functional tests to pass, due to a change in how urllib3's header management works.

I made this change because I need both [moto](https://pypi.python.org/pypi/moto/0.3.1) and [elasticsearch](https://pypi.python.org/pypi/elasticsearch) libraries, moto depends on HTTPretty, and the current version of elasticsearch requires urllib3 1.9.1, causing a dependency conflict.

Let me know if there's any improvements or changes you want to see in this pull request.