Fix selectOnBlur and closeOnSelect combination

This fixes an infinite loop that used to be caused when both
`closeOnSelect` and `selectOnClose` used to be combined, because they
both were listening to events triggered by the other one. The problem
was that `selectOnClose` was triggering `select` events for data objects
which had already been selected. This problem was solved by checking if
the data object was already selected before trying to select it again.

This closes https://github.com/select2/select2/pull/3751.
This closes https://github.com/select2/select2/issues/3169.

src/js/select2/core.js

@@ -357,7 +357,7 @@ define([
       'select': 'selecting',
       'unselect': 'unselecting'
     };
-    
+
     if (args === undefined) {
       args = {};
     }

src/js/select2/dropdown/selectOnClose.js

@@ -16,12 +16,23 @@ define([
   SelectOnClose.prototype._handleSelectOnClose = function () {
     var $highlightedResults = this.getHighlightedResults();
 
+    // Only select highlighted results
     if ($highlightedResults.length < 1) {
       return;
     }
 
+    var data = $highlightedResults.data('data');
+
+    // Don't re-select already selected resulte
+    if (
+      (data.element != null && data.element.selected) ||
+      (data.element == null && data.selected)
+    ) {
+      return;
+    }
+
     this.trigger('select', {
-        data: $highlightedResults.data('data')
+        data: data
     });
   };