Use getTile() instead of imgDiv to get the correct element

The correct way to get the markup of a tile is to use getTile() - the
imgDiv will only be available if the tile does not use a frame.
.olTileReplacing is on the parent of .olTileImage when using frame
Merge pull request #975 from ahocevar/gettile

mergeNewParams does not work with POST requests. r=@bartvde