Change request to make keep alive more human

Not sure if this is the right way & design to do this:
* I added a new property to request and session
* I used existing merge function to assure that if something is set in request parameter, it will overides sessions´s ones.
* I keep it disable by deafault as https://github.com/kennethreitz/requests/issues/2169#issuecomment-53496634 stated.

More info in https://github.com/kennethreitz/requests/issues/2169 thread.