Changes to mob spawning algorithm (Mobs don't require 'Air'-block for spawning cycle)

Previously, mobs did not spawn if there were no air blocks above the solid block chosen as the initial part of the spawn algorithm.

As of 14w25b snapshot, this is no longer the case. There is no obvious mention of such a change in the notes, hence filing this as a bug.

Creating a flat world with rail as the top surface results in no mobs spawning pre 14w25. From 14w25 and onwards, mobs will spawn despite the lack of completely clear spaces previously required to "seed" spawning.