Fix truncate on models

According to docs array or object might be returned, so we need to add options. See https://docs.mongodb.com/php-library/v1.5/reference/method/MongoDBCollection-drop/

Author: divine

Diff for: src/Jenssegers/Mongodb/Query/Builder.php

@@ -700,7 +700,11 @@ public function from($collection, $as = null)
      */
     public function truncate()
     {
-        $result = $this->collection->drop();
+        $options = [
+            'typeMap' => ['root' => 'object', 'document' => 'object'],
+        ];
+
+        $result = $this->collection->drop($options);
 
         return (1 == (int) $result->ok);
     }

Diff for: tests/QueryBuilderTest.php

@@ -175,7 +175,8 @@ public function testDelete()
     public function testTruncate()
     {
         DB::collection('users')->insert(['name' => 'John Doe']);
-        DB::collection('users')->truncate();
+        $result = DB::collection('users')->truncate();
+        $this->assertEquals(1, $result);
         $this->assertEquals(0, DB::collection('users')->count());
     }