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/