Description:
------------
xmlrpc_encode_request XML-escapes literally without first converting 
to UTF-16 as required.

Once fixed the default option should also be changed from escaping 
non-ascii, to escaping non-encodable

Reproduce code:
---------------
xmlrpc_encode_request('method', '€', array('encoding'=>'utf-8'));

Expected result:
----------------
<?xml version="1.0" encoding="utf-8"?>
<methodCall>
<methodName>method</methodName>
<params>
 <param>
  <value>
   <string>&#8364;</string>
  </value>
 </param>
</params>
</methodCall>


Actual result:
--------------
<?xml version="1.0" encoding="utf-8"?>
<methodCall>
<methodName>method</methodName>
<params>
 <param>
  <value>
   <string>&#226;&#130;&#172;</string>
  </value>
 </param>
</params>
</methodCall>