Updated Invoicing APIs

- Updated Model objects.
- Updated Samples.
- Updated Tests.

tests/PayPal/Test/Api/InvoiceNumberTest.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace PayPal\Test\Api;
+use PayPal\Api\InvoiceNumber;
+
+
+/**
+ * Class Cost
+ *
+ * @package PayPal\Test\Api
+ */
+class InvoiceNumberTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * Gets Json String of Object Cost
+     * @return string
+     */
+    public static function getJson()
+    {
+        return '{"number":"1234"}';
+    }
+
+    /**
+     * Gets Object Instance with Json data filled in
+     * @return InvoiceNumber
+     */
+    public static function getObject()
+    {
+        return new InvoiceNumber(self::getJson());
+    }
+
+
+    /**
+     * Tests for Serialization and Deserialization Issues
+     * @return InvoiceNumber
+     */
+    public function testSerializationDeserialization()
+    {
+        $obj = new InvoiceNumber(self::getJson());
+        $this->assertNotNull($obj);
+        $this->assertNotNull($obj->getNumber());
+        $this->assertEquals(self::getJson(), $obj->toJson());
+        return $obj;
+    }
+
+    /**
+     * @depends testSerializationDeserialization
+     * @param InvoiceNumber $obj
+     */
+    public function testGetters($obj)
+    {
+        $this->assertEquals($obj->getNumber(), "1234");
+    }
+    
+}