Only extend database when it is actually resolved.

src/Jenssegers/Mongodb/MongodbServiceProvider.php

@@ -25,9 +25,11 @@ class MongodbServiceProvider extends ServiceProvider {
     public function register()
     {
         // Add a mongodb extension to the original database manager
-        $this->app['db']->extend('mongodb', function($config)
-        {
-            return new Connection($config);
+        $this->app->resolving('db', function($db) {
+            $db->extend('mongodb', function($config)
+            {
+                return new Connection($config);
+            });
         });
     }