valueAsArray to handle passed arrays

In the test suite sometimes a mutableValue array is passed when `multiple == false`. Using [].concat rather than an array literall allows valueAsArray to handle these cases.   [].concat could handle the case when `multiple == true` as well.

src/components/Select.vue

@@ -952,7 +952,7 @@
         if (this.multiple) {
           return this.mutableValue
         } else if (this.mutableValue) {
-          return [this.mutableValue]
+          return [].concat(this.mutableValue)
         }
 
         return []