Dialog: Add type="button" to the close button. Fixed #9312: Dialog: clos...

...es on enter in textbox in IE.

I cannot come up with a way to simulate this behavior in IE in a unit test. This is the test that I tried writing:

```
test( "#9312: dialog closes on enter keypresses in textboxes", function() {
	expect( 1 );
	var input = $( "<input>" ),
		dialog = $( "<div></div>" )
			.html( input )
			.dialog();

	input.simulate( "keydown", { keyCode: $.ui.keyCode.ENTER });

	ok( dialog.dialog( "isOpen" ) );
	dialog.dialog( "destroy" );
});
```

Except this doesn't force the close button's click event to fire. If I'm missing something obvious let me know. I went with an explicit inline comment for now.