Exposed a method to change cached sessionId

Needed a way to change the already set session id during login and make further requests. This is helpful when a previous sessionId is already available and I don't wan't to create a new session. The previous sessionId is stored too, just in case we need to switch back, though I haven't implemented switching back.

Added a small test too. I'm thinking of implementing a better test with mocha when I have some time.

Let me know if this is helpful.
add a `return this;` at the end of this function, so it's chainable