Fix players spawning above portals. Fixes BUKKIT-3542.

Vanilla does not check for blocks in which the player could suffocate when changing dimension, so portals will happily spawn players in blocks when using a portal under certain circumstances. However, we currently check for these instances and move the player up until they will not suffocate. This means that players can sometimes be taken to above the target portal, making it seem as if a portal was not created. Instead, we now disable this suffocation check when moveToWorld is called from changeDimension, mirroring vanilla behaviour more accurately.