processing input value attributes the same as input value properties

@sdelaney this handles the way angular updates the value. In multiselect we do `value={{..some exp..}}` and angular watches the expression and calls `$attr.$set('value', 'exp result')` when it changes.  `$attr.$set` uses the jQuery `$.attr` and NOT `$.prop`. Previously jquery-placeholder only handled the value property and not attribute changes...