fix($injector): check if a fn is an array explicitly

This change makes the code easier to read and also fixes a compatibility issue
with opal.js which pollutes the global state by setting $inject property on
Array prototype

Closes #7904
Closes #2653

src/auto/injector.js

@@ -762,8 +762,7 @@ function createInjector(modulesToLoad) {
           : getService(key)
         );
       }
-      if (!fn.$inject) {
-        // this means that we must be an array.
+      if (isArray(fn)) {
         fn = fn[length];
       }