fix(uiSelectMultiple): Allow duplicates in $select.selected

add track by $index to $select.selected ng-repeat

Closes #1688

Author: sh977218

Diff for: src/bootstrap/match-multiple.tpl.html

@@ -1,6 +1,6 @@
 <span class="ui-select-match">
-  <span ng-repeat="$item in $select.selected">
-    <span 
+  <span ng-repeat="$item in $select.selected track by $index">
+    <span
       class="ui-select-match-item btn btn-default btn-xs"
       tabindex="-1"
       type="button"

Diff for: src/select2/match-multiple.tpl.html

@@ -4,7 +4,7 @@
   do not work: [class^="select2-choice"]
 -->
 <span class="ui-select-match">
-  <li class="ui-select-match-item select2-search-choice" ng-repeat="$item in $select.selected" 
+  <li class="ui-select-match-item select2-search-choice" ng-repeat="$item in $select.selected track by $index"
       ng-class="{'select2-search-choice-focus':$selectMultiple.activeMatchIndex === $index, 'select2-locked':$select.isLocked(this, $index)}"
       ui-select-sort="$select.selected">
       <span uis-transclude-append></span>