WPRichTextView: Forcing a draw cycle

Fixes #2347 

Alternative to this workaround [is outlined here](http://stackoverflow.com/questions/5198155/not-all-tiles-redrawn-after-catiledlayer-setneedsdisplay).

This extra call to *setNeedsDisplayInBounds:* should *NOT* be required, since it's already being called *DTAttributedTextView*'s *relayoutText* method. However, there seems to be a glitch related to the CATiledLayer usage, in which the tiles might not be redrawn.

By delaying the *setNeedsDisplay* call, i've managed to verify that the issue disappears.

Thanks!

/cc @aerych