Check to see if local file exists before opening

[palexander]

The RDF::Util::File.open_file method was throwing an exception when the local file did not exist. This modification will check to see if it exists, avoiding the exception and allowing open-uri to download the file.

I'm also getting errors when running the specs. Many of them look like formatting problems, EX:

40) JSON::LD test suite toRdf toRdf-0026-in.jsonld: Test creation of multiple types
Failure/Error: quads.sort.join("").should produce(sorted_expected, t.debug)
  Expected: _:b0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Class> .
  _:b0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Resource> .

  Actual  : _:t0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Class> .
  _:t0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Resource> .

[gkellogg]

Thanks a lot! The need for this override may go away, due to recent changes in RDF.rb. There's also much more work to be done to sync up with recent spec changes, which I'm in the middle of, but am burried under a lot of other stuff.