Merge pull request #2789 from RustyToms/drop_above_dropHeight_fix

recalculate $dropdown height after drop-above class added

select2.js

@@ -1325,10 +1325,11 @@ the specific language governing permissions and limitations under the Apache Lic
             };
 
             if (above) {
-                css.top = offset.top - dropHeight;
-                css.bottom = 'auto';
                 this.container.addClass("select2-drop-above");
                 $dropdown.addClass("select2-drop-above");
+                dropHeight = $dropdown.outerHeight(false);
+                css.top = offset.top - dropHeight;
+                css.bottom = 'auto';
             }
             else {
                 css.top = dropTop;