Allow the server to return a partial state from fetch

[wyuenho]

This line in parseState is overwriting the state with undefined if the server doesn't return all the values, which may result in weird states from time to time.

https://github.com/wyuenho/backbone-pageable/blob/1.1.7/lib/backbone-pageable.js#L1031

Since mosts states are recalculated on the client side anyway, this method should allow the server to return a partial state object so users don't have to explicitly override this method.