<p:serialization> method attribute parsed incorrectly if there is a default namespace

In my pipelines, I like to use the XProc namespace as the default namespace. For the most part, this works fine, except for one bug: the value of the `method` attribute on `p:serialization` elements is interpreted as being in that default namespace!

This change fixes that. If the method name does not have a prefix, then it is now interpreted as being in no namespace, [as per the spec](http://www.w3.org/TR/xproc/#syntax-summaries).