Fixed a bug which removes the xmlns attributes in metadata element

The current xoai removes the `xmlns` attributes specified at metadata root element in its xslt. This patch fixes this problem.

For example with oai_dc,

Current xaoi-3.2.7 writes as follows:
````
<oai_dc:dc xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/
               http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
````
xaoi-3.2.7 with this patch writes:
````
<oai_dc:dc xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/"
           xmlns:doc="http://www.lyncode.com/xoai"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns:dc="http://purl.org/dc/elements/1.1/"
           xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/
               http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
````