Fixed issue found while try to troubleshoot ruby-saml with jruby

when XSD has DOCTYPE to DTD, not able to load the file.

and the ruby script is run without path, NullPointerException got thrown.

Test to demostrate https://github.com/sparklemotion/nokogiri/pull/791
Merge pull request #912 from patcheng/jruby_schema_validation

fixed JRuby schema validation when XSD has DOCTYPE to DTD