[Bleeding] Return all blocks involved in PortalCreateEvent. Fixes BUKKIT-5464

Due to changes in how portals were created in Minecraft 1.7, the code that
CraftBukkit previously used to find the blocks involved in the
PortalCreateEvent no longer detected all blocks. Additionally, in the process
of the update to 1.7, a missed diff resulted in some blocks that were found not
being properly added to the blocklist. This commit corrects that missed diff,
while also adding a check to ensure that the top and bottom of the portal frame
are included in the blocklist.