Ensure prefixes are not repeated when using specific namespaces and also some prefixes are read from turtle files

[DaniFdezAlvarez]

Example. If I use this dict in default_namespaces:

{ "http://example.org/": "ex", "http://www.w3.org/XML/1998/": "xml", "http://www.w3.org/1999/02/": "rdf", "http://www.w3.org/2000/01#": "rdfs", "http://www.w3.org/2001#": "xsd", "http://xmlns.com/foaf/": "foaf" }

and also I parse turtle files with these other namespaces:

@prefix rdf: http://www.w3.org/1999/02/22-rdf-syntax-ns# . @prefix rdfs: http://www.w3.org/2000/01/rdf-schema# . @prefix ex: http://example.org/ . @prefix foaf: http://xmlns.com/foaf/0.1/ . @prefix xsd: http://www.w3.org/2001/XMLSchema# .

Then my prefix declarations in ShExC look like this:

PREFIX ex: http://example.org/ PREFIX xml: http://www.w3.org/XML/1998/ PREFIX rdf: http://www.w3.org/1999/02/ PREFIX rdfs: http://www.w3.org/2000/01# PREFIX xsd: http://www.w3.org/2001# PREFIX foaf: http://xmlns.com/foaf/ PREFIX : http://weso.es/shapes/ PREFIX xml: http://www.w3.org/XML/1998/namespace/ PREFIX rdf: http://www.w3.org/1999/02/22-rdf-syntax-ns# PREFIX rdfs: http://www.w3.org/2000/01/rdf-schema# PREFIX xsd: http://www.w3.org/2001/XMLSchema# PREFIX foaf: http://xmlns.com/foaf/0.1/