Minor adjustments and cleanup, including normalizing the value to a string when setti...

Minor adjustments and cleanup, including normalizing the value to a string when setting( list of changes below )

- Normalize set value to string to synchronize return type cross-browser

- Add style attrHook to propHooks to support style getting in all browsers for both attr and prop

- Extend the selected propHook instead of overriding a possible set function

- Remove selected propHook TODO since there is no selected content attribute and it should return null