Move fake session to HTTPClient

The fake session object is to prevent a cyclical dependency between
HTTPClient and the session from leaving hanging session objects around.

This is still necessary if you construct a client the old way however if
you are using the session properly then there is no cyclical dependency
and so we shouldn't prevent people using the connection pooling
advantages of the session.

Related-Bug: #1282089
Change-Id: Ifca2c7ddd95a81af01ee43246ecc8e74abf95602
Merge "Move fake session to HTTPClient"