Also decode entities in namespace URIs if entity substitution wasn't
requested. This should fix some corner cases when comparing namespace
URIs. The Namespaces in XML 1.0 spec says:

> In a namespace declaration, the URI reference is the normalized value
> of the attribute, so replacement of XML character and entity
> references has already been done before any comparison.

Make the serialization code escape special characters in namespace URIs
like in attribute values. This fixes serialization if entities were
substituted when parsing.

Fixes libxslt#106