[core] Fix http client pool by correctly mounting Transport Adapter to a prefix.

Requests enables users to create and use their own Transport Adapters that provide specific functionality. Once created, a Transport Adapter can be mounted to a Session object, along with an indication of which web services it should apply to.

>>> s = requests.Session()
>>> s.mount('http://www.github.com', MyAdapter())
The mount call registers a specific instance of a Transport Adapter to a prefix. Once mounted, any HTTP request made using that session whose URL starts with the given prefix will use the given Transport Adapter.

In past we incorrectly prefix to "http://" or "https://" which can cause 401 errors.