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

src/event.js

@@ -746,10 +746,10 @@ 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