Description:
------------
When extending HttpMessage and having an array property I run 
into "PHP Fatal error:  Cannot access HttpMessage properties 
by reference or array key/index"

Reproduce code:
---------------
<?php

class ChildMessage extends HttpMessage
{

    public $properties = array();

}

$child = new ChildMessage;
$child->properties['foo'] = 'bar';

?>


Expected result:
----------------
ChildMessage::$properties['foo'] to equal 'bar'

Actual result:
--------------
PHP Fatal error:  Cannot access HttpMessage properties by 
reference or array key/index in /home/hodsdon/bug.php on line 
11