Fixes to Json conversion logic for empty objects

- If the object is empty, an empty json object is generated on conversion
- Fixes #223

tests/PayPal/Test/Common/PayPalModelTest.php

@@ -184,6 +184,27 @@ class PayPalModelTest extends PHPUnit_Framework_TestCase
 
     }
 
+    /**
+     * @test
+     */
+    public function testEmptyObject()
+    {
+        $child = new SimpleModelTestClass();
+        $child->setField1(null);
+
+        $parent = new ContainerModelTestClass();
+        $parent->setField1("parent");
+        $parent->setNested1($child);
+
+        $this->assertEquals('{"field1":"parent","nested1":{}}',
+            $parent->toJSON());
+
+        $parentCopy = new ContainerModelTestClass();
+        $parentCopy->fromJson($parent->toJSON());
+        $this->assertEquals($parent, $parentCopy);
+
+    }
+
     /**
      * @test
      */