Fix a TextFied color bug in iOS7

This fix a bug in iOS 7 where a white TextField (0xffffff) becomes yellow (0xffff00) after 3 seconds (if cacheAsBitmap is disabled).
I suppose this is a bug in an optimization of Safari but making this little change solved the problem.
Merge pull request #55 from xavierhainaux/patch-1

Fix a TextFied color bug in iOS7