Slider: On Opera Mobile event.which is undefined. Accept that and get…

… slidin'. Fixes #5903.

(cherry picked from commit b90361b)

js/widgets/forms/slider.js

@@ -261,7 +261,7 @@ $.widget( "mobile.slider", $.mobile.widget, $.extend( {
 	_sliderVMouseDown: function( event ) {
 		// NOTE: we don't do this in refresh because we still want to
 		//       support programmatic alteration of disabled inputs
-		if ( this.options.disabled || !( event.which === 1 || event.which === 0 ) ) {
+		if ( this.options.disabled || !( event.which === 1 || event.which === 0 || event.which === undefined ) ) {
 			return false;
 		}
 		if ( this._trigger( "beforestart", event ) === false ) {

tests/unit/slider/slider_events.js

@@ -375,7 +375,7 @@
 	});
 
 	asyncTest( "drag should start only when clicked with left button", function(){
-		expect( 4 );
+		expect( 5 );
 
 		var control = $( "#mousedown-which-events" ),
 			widget = control.data( "mobile-slider" ),
@@ -405,6 +405,15 @@
 			function( result ) {
 				deepEqual( result.slidestart.timedOut, false, "slider did emit 'slidestart' event upon left button press" );
 				event = $.Event( "mousedown", { target: handle[ 0 ] } );
+				event.which = undefined;
+				slider.trigger( event );
+			},
+			{
+				slidestart: { src: control, event: "slidestart" + eventNs + "1" }
+			},
+			function( result ) {
+				deepEqual( result.slidestart.timedOut, false, "slider did emit 'slidestart' event upon undefined button press" );
+				event = $.Event( "mousedown", { target: handle[ 0 ] } );
 				event.which = 2;
 				slider.trigger( event );
 			},