Change multiselect cache structure

The exisiting structure used an Object as a cache where the keys are the
values of option elements, and values are the HTML content of the option
elements. In Javascript, if a numeric value in the form of a string is
assigned as a key, it gets converted to an integer. Optimization
routines would then order the object to ensure faster access to
elements. For example:

cache = {}
cache['2'] = "two"
cache['1'] = "one"
console.log(cache[2])  #=> "two"
console.log(cache[1])  #=> "one"
console.log(cache)     #=> {1: "one", 2: "two"}

Note the coercion of strings to ints above. This messes with the
ordering of multiselect options list whenever there is a user input. To
avoid this from happening, the keys need to have a string that can't be
coerced automatically, and then preserve the value of the option
element.

I chose to use an object that stores the option value and the option
HTML as the value of the cache and a string of the format 'o_<option
value>' as the key. This ensures that the insertion order is preserved.
This is the new structure:

cache = {
  'o_271': { id: 271, value: 'CartItem railsadminteam#271'},
  'o_270': { id: 270, value: 'CartItem railsadminteam#270}'
}

app/assets/javascripts/rails_admin/ra.filtering-multiselect.js

@@ -180,10 +180,10 @@
 
       this.element.find("option").each(function(i, option) {
         if (option.selected) {
-          widget._cache[option.value] = option.innerHTML;
+          widget._cache['o_' + option.value] = {id: option.value, value: option.innerHTML};
           $(option).clone().appendTo(widget.selection).attr("selected", false).attr("title", $(option).text());
         } else {
-          widget._cache[option.value] = option.innerHTML;
+          widget._cache['o_' + option.value] = {id: option.value, value: option.innerHTML};
           $(option).clone().appendTo(widget.collection).attr("selected", false).attr("title", $(option).text());
         }
       });
@@ -206,7 +206,8 @@
         if (!this.options.xhr) {
           for (i in this._cache) {
             if (this._cache.hasOwnProperty(i)) {
-              matches.push({id: i, label: this._cache[i]});
+              option = this._cache[i];
+              matches.push({id: option.id, label: option.value});
             }
           }
         }
@@ -231,8 +232,9 @@
           query = new RegExp(query + '.*', 'i');
 
           for (i in this._cache) {
-            if (this._cache.hasOwnProperty(i) && query.test(this._cache[i])) {
-              matches.push({id: i, label: this._cache[i]});
+            if (this._cache.hasOwnProperty(i) && query.test(this._cache[i]['value'])) {
+              option = this._cache[i];
+              matches.push({id: option.id, label: option.value});
             }
           }