pass defaults as a param so spread can be used

src/components/Select.vue

@@ -13,7 +13,7 @@
               :deselect="deselect"
               :multiple="multiple"
               :disabled="disabled">
-          <span :key="getOptionKey(option)" :class="getSelectedOptionClasses(option)">
+          <span :key="getOptionKey(option)" :class="getSelectedOptionClasses(option, ['vs__selected'])">
             <slot name="selected-option" v-bind="normalizeOptionForSlot(option)">
               {{ getOptionLabel(option) }}
             </slot>
@@ -313,9 +313,10 @@
          * @see https://vuejs.org/v2/guide/class-and-style.html
          *
          * @param option {Object|String|Number} An option that has been selected
+         * @param defaults {Array} Array of default classes ['vs__selected']
          * @return {Array|Object|String} Any valid VueJS class binding format.
          */
-        default: option => ['vs__selected']
+        default: (option, defaults) => defaults
       },
 
       /**

tests/unit/Selecting.spec.js

@@ -221,10 +221,10 @@ describe("VS - Selecting Values", () => {
     it("can add a custom class using the getSelectedOptionClasses prop", () => {
       const Select = mountDefault({
         value: 'one',
-        getSelectedOptionClasses: option => ['vs__selected', `vs__selected--${option}`]
+        getSelectedOptionClasses: (option, defaults) => [...defaults, `vs__selected--${option}`]
       });
-      
-      expect(Select. contains('.vs__selected')).toBe(true);
+
+      expect(Select.contains('.vs__selected')).toBe(true);
       expect(Select. contains('.vs__selected--one')).toBe(true);
     });
   });