attributeBindings is frozen, slice it before push (#59)

As noted in the Ember v2.11 release notes, concatenated properties such
as attributeBindings are frozen in debug builds. This means we cannot
push directly onto the attributeBindings array without first copying it
via slice.

Without this change, users may see errors like this when the
`attributeBindings.push()` call is made:

Uncaught TypeError: Can't add property 2, object is not extensible

References:

emberjs/ember.js#14389
emberjs/ember.js#14601

addon/utils/bind-data-test-attributes.js

@@ -37,6 +37,8 @@ export default function bindDataTestAttributes(component) {
   let attributeBindings = component.getWithDefault('attributeBindings', []);
   if (!Ember.isArray(attributeBindings)) {
     attributeBindings = [attributeBindings];
+  } else {
+    attributeBindings = attributeBindings.slice();
   }
 
   dataTestProperties.forEach(it => attributeBindings.push(it));