After fast-case assignment to a pixel array the original value assigned is now returned. Before the un-tagged smi value was returned causing crashes.

BUG=22913
TEST=cctest/test-api/PixelArray
Review URL: http://codereview.chromium.org/248033

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2985 ce2b1a6d-e550-0410-aec6-3dcde31c8c00