Tilemap.swap() called twice should not flip values

Repeat calls to swap() should not affect index values a second time.
"swap(1,2); swap(1,2)" should not be the same as "swap(2,1)."

(This also affects Phaser 1.1.6.)
Merge pull request #1 from videlais/patch-1

Tilemap.swap() called twice should not flip values