Split record_to_xml to create a intermediate function returning ElemenTree node

Hi, 
I'm playing with MARC records and generating a MARC21 XML file including XML decorator, collection tag and so.  I call to function record_to_xml and parse back the string to ElementTree node but looking inside function I see it's generating a ElementTree node and  serialize it  just in the return sentence. 

I split  function into two smaller ones, allowing build a xml document very easy without overhead of serialization/deserialization each node. 

Modification keep old funcionality and all test pass OK.

I hope this mod will be useful to you as it's for me.

Regards.
 Manuel.