add custom fields

src/PropertyBag.php

@@ -17,6 +17,7 @@ class PropertyBag implements \IteratorAggregate
      * @var array
      */
     protected $elements = [];
+    protected $xprops = [];
 
     /**
      * Creates a new Property with $name, $value and $params.
@@ -60,6 +61,10 @@ class PropertyBag implements \IteratorAggregate
     public function add(Property $property)
     {
         $name = $property->getName();
+        if ('X-' == substr($name, 0, 2)) {
+          $this->xprops[] = $property;
+          return $this;
+        }
 
         if (isset($this->elements[$name])) {
             throw new \Exception("Property with name '{$name}' already exists");
@@ -72,6 +77,6 @@ class PropertyBag implements \IteratorAggregate
 
     public function getIterator()
     {
-        return new \ArrayObject($this->elements);
+        return new \ArrayObject($this->elements + $this->xprops);
     }
 }