Event: Don't break focus triggering after .on(focus).off(focus)

The `_default` function in the special event settings for focus/blur has
always returned `true` since gh-4813 as the event was already being fired
from `leverageNative`. However, that only works if there's an active handler
on that element; this made a quick consecutive call:

```js
elem.on( "focus", function() {} ).off( "focus" );
```

make subsequent `.trigger( "focus" )` calls to not do any triggering.

The solution, already used in a similar `_default` method for the `click` event,
is to check for the `dataPriv` entry on the element for the focus event
(similarly for blur).

Fixes gh-4867
Closes gh-4885

(cherry picked from commit e539bac)

src/event.js

@@ -778,10 +778,10 @@ jQuery.each( { focus: "focusin", blur: "focusout" }, function( type, delegateTyp
 			return true;
 		},
 
-		// Suppress native focus or blur as it's already being fired
-		// in leverageNative.
-		_default: function() {
-			return true;
+		// Suppress native focus or blur if we're currently inside
+		// a leveraged native-event stack
+		_default: function( event ) {
+			return dataPriv.get( event.target, type );
 		},
 
 		delegateType: delegateType

test/unit/event.js

@@ -3297,6 +3297,22 @@ QUnit.test( "focus change during a focus handler (gh-4382)", function( assert )
 	} );
 } );
 
+QUnit.test( "trigger(focus) works after .on(focus).off(focus) (gh-4867)", function( assert ) {
+	assert.expect( 1 );
+
+	var input = jQuery( "<input />" );
+
+	input.appendTo( "#qunit-fixture" );
+
+	input
+		.on( "focus", function() {} )
+		.off( "focus" );
+
+	input.trigger( "focus" );
+
+	assert.equal( document.activeElement, input[ 0 ], "input has focus" );
+} );
+
 // TODO replace with an adaptation of
 // https://github.com/jquery/jquery/pull/1367/files#diff-a215316abbaabdf71857809e8673ea28R2464
 ( function() {