Resolving entity values with nested entities

[filippodebortoli]

I am trying to get RdfXmlParser to parse a file that contains entities whose defined in terms of other entities, as rdf and rdfs in the following example:

<?xml version="1.0"?>

<!DOCTYPE rdf:RDF [
  <!ENTITY ex "http://example.com/">
  <!ENTITY w3 "http://www.w3.org">
  <!ENTITY rdf "&w3;/1999/02/22-rdf-syntax-ns#">
  <!ENTITY rdfs "&w3;/2000/01/rdf-schema#">
  <!ENTITY xsd "&w3;/2001/XMLSchema#">
]>

<rdf:RDF xmlns:ex2="&ex;2/" xmlns:rdf="&rdf;">
	<rdf:Description rdf:about="&ex;foo">
	    <ex2:test rdf:datatype="&xsd;string">bar</ex2:test>
	</rdf:Description>
</rdf:RDF>

It seems that the parser is not performing entity resolution while parsing doctypes.
I have to admit that I am unsure whether the code above is well-formed, I encountered a situation of this kind while working with a seemingly established ontology.
I am trying to add this capability to the parser by modifying parser::parse_doctype().
I am going to address this issue in a pull request.

Am I missing some capabilities of RdfXmlParser that would allow me to get the parser to work on this example without changing code?