fix(association-select): use model.bind instead of value.bind

BREAKING CHANGE: Can break module for users that expect a string when given `undefined`, `null` or `[]`.

src/component/association-select.html

@@ -1,6 +1,6 @@
 <template>
   <select class="form-control" value.bind="value" multiple.bind="multiple" disabled.bind="disabled">
-    <option selected disabled.bind="!hidePlaceholder && !selectablePlaceholder" value.bind="placeholderValue" show.bind="!hidePlaceholder" t="[html]${placeholderText || '- Select a value -'}">${placeholderText || '- Select a value -'}</option>
+    <option selected disabled.bind="!hidePlaceholder && !selectablePlaceholder" model.bind="placeholderValue" show.bind="!hidePlaceholder" t="[html]${placeholderText || '- Select a value -'}">${placeholderText || '- Select a value -'}</option>
     <option model.bind="option.id" repeat.for="option of options">${option[property]}</option>
   </select>
 </template>