util.from_n3() is broken wrt. Literals with datatypes

[joernhees]

In [1]: import rdflib
INFO:rdflib:RDFLib Version: 4.2.1-dev

In [2]: rdflib.util.from_n3('"42"^^<http://www.w3.org/2001/XMLSchema#integer>')
WARNING:rdflib.term:<http://www.w3.org/2001/XMLSchema#integer> does not look like a valid URI, trying to serialize this will break.
Out[2]: rdflib.term.Literal(u'42', datatype=rdflib.term.URIRef(u'<http://www.w3.org/2001/XMLSchema#integer>'))

problem here is that the < is passed into URIRef() as part of the URI.

In [3]: rdflib.util.from_n3('"42"^^xsd:integer')
Out[3]: rdflib.term.Literal(u'42', datatype=rdflib.term.URIRef(u'xsd:integer'))

problem is that standard xsd datatypes aren't resolved correctly but instead passed into URIRef() as is.

In [4]: rdflib.util.from_n3('"42"^^http://www.w3.org/2001/XMLSchema#integer')
Out[4]: rdflib.term.Literal(u'42', datatype=rdflib.term.URIRef(u'http://www.w3.org/2001/XMLSchema#integer'))

In [5]: rdflib.Literal(42) == rdflib.util.from_n3('"42"^^http://www.w3.org/2001/XMLSchema#integer')
Out[5]: True

this is actually invalid n3 but the only thing that returns the expected Literal

More fun (round-tripping):

In [6]: rdflib.util.from_n3(rdflib.Literal(42).n3()).n3()
WARNING:rdflib.term:<http://www.w3.org/2001/XMLSchema#integer> does not look like a valid URI, trying to serialize this will break.
Out[6]: u'"42"^^<<http://www.w3.org/2001/XMLSchema#integer>>'