Force the data-* attributes to be parsed

There was a change made that switched us from using `$.data` and
`$.fn.data` internally to using an internal data store (managed
through internal utilities). This had the unfortunate side effect
of breaking the automatic loading of data-* options in versions of
jQuery other than 1.x, which included anything that would be
considered modern jQuery. While the change was made and approved
in good faith, all of the tests passed and the docs pages appeared
to be working, the tests really failed when running on newer versions
of jQuery. This was confirmed when we started running automated tests
against both versions, which confirmed the bug that others have been
seeing for a while.

The change was made becuase calling `$.fn.data` on an element which
contains a reference to itself in the internal jQuery data cache
would cause a stack overflow. This bug was well documented at the
following GitHub ticket and was resolved by no longer using
`$.fn.data`: #4014

Unfortunately because `$.fn.data` was no longer being called in a
way such that all of the data attributes would be dumped out, we
needed to find a replacement. The substitute that was given in the
original bug fix worked when the data cache was fully primed, but
we never primed it anywhere so it actually failed in the general
case. That meant we needed to find a way to manually prime it,
which is exactly what this change does.

src/js/select2/options.js

@@ -79,20 +79,43 @@ define([
 
       $e.attr('ajax--url', Utils.GetData($e[0], 'ajaxUrl'));
       Utils.StoreData($e[0], 'ajax-Url', Utils.GetData($e[0], 'ajaxUrl'));
-
     }
 
     var dataset = {};
 
+    function upperCaseLetter(_, letter) {
+      return letter.toUpperCase();
+    }
+
+    // Pre-load all of the attributes which are prefixed with `data-`
+    for (var attr = 0; attr < $e[0].attributes.length; attr++) {
+      var attributeName = $e[0].attributes[attr].name;
+      var prefix = 'data-';
+
+      if (attributeName.substr(0, prefix.length) == prefix) {
+        // Get the contents of the attribute after `data-`
+        var dataName = attributeName.substring(prefix.length);
+
+        // Get the data contents from the consistent source
+        // This is more than likely the jQuery data helper
+        var dataValue = Utils.GetData($e[0], dataName);
+
+        // camelCase the attribute name to match the spec
+        var camelDataName = dataName.replace(/-([a-z])/g, upperCaseLetter);
+
+        // Store the data attribute contents into the dataset since
+        dataset[camelDataName] = dataValue;
+      }
+    }
+
     // Prefer the element's `dataset` attribute if it exists
     // jQuery 1.x does not correctly handle data attributes with multiple dashes
     if ($.fn.jquery && $.fn.jquery.substr(0, 2) == '1.' && $e[0].dataset) {
-      dataset = $.extend(true, {}, $e[0].dataset, Utils.GetData($e[0]));
-    } else {
-      dataset = Utils.GetData($e[0]);
+      dataset = $.extend(true, {}, $e[0].dataset, dataset);
     }
 
-    var data = $.extend(true, {}, dataset);
+    // Prefer our internal data cache if it exists
+    var data = $.extend(true, {}, Utils.GetData($e[0]), dataset);
 
     data = Utils._convertData(data);