fix typo in Serializer and update tests

Author: PapiHack

src/Utils/Serializer.php

@@ -34,7 +34,7 @@ public function toJSONString()
 
     public function toXMLString()
     {
-        return $this->arrayToXml($this->data, '<custom_fields/>');
+        return $this->arrayToXml($this->data, '<custom_field/>');
     }
 
     public function toQueryString()

tests/Unit/Utils/SerializerTest.php

@@ -20,6 +20,6 @@
  });
 
  it('Should return a XML', function() {
-    $this->assertXmlStringEqualsXmlString("<custom_fields><nom>papi</nom><job>coder</job></custom_fields>", 
+    $this->assertXmlStringEqualsXmlString("<custom_field><nom>papi</nom><job>coder</job></custom_field>", 
     (new Serializer(['nom' => 'papi', 'job' => 'coder']))->toXMLString());
  });