Tentative fix for issue 416: TileMapServiceImageryProvider extent 

The fix I implemented intersect
the extent of each tile (of the imagery service provider tiling scheme) which
contains some part of the imagery with the imagery extent.
However I noticed that the extent of each cached imagery (an imagery for each
tile) has to be the extent of the tile and not the extent of the intersection.
I committed also a Sandcastle example (Test Imagery Layer) that I utilized for
testing.