Revert instanceOf check

Despite what the documentation says about `ReflectionClass::instanceOf()`, (or `->isIntance()`), the result seems to be unreliable.

Reverting back to fully instantiating the object to see if it is an extension.

Resolves #147

src/Generators/AbstractTagGenerator.php

@@ -215,7 +215,7 @@ protected function generateOwnerTags()
         if ($reflection->isAbstract()) {
             return;
         }
-        if ($reflection instanceof Extension) {
+        if (Injector::inst()->get($this->className) instanceof Extension) {
             $owners = iterator_to_array($this->getOwnerClasses($className));
             $owners[] = $this->className;
             $tagString = sprintf('\\%s $owner', implode("|\\", array_values($owners)));