RELAX NG Validator fails to handle XML documents with XML declaration

The RELAX NG validator currently does not handle XML documents with an XML declaration, such as <?xml version="1.0" encoding="UTF-8"?>.

As a first step to address this issue, I suggest adding the following test case to test/test_validation_rng.rb. This test case checks that the validator can successfully validate an XML document with a declaration. I haven't yet developed an implementation solution, but this test will help ensure that we cover this scenario:

def test_document_with_declaration
  rng = <<-XML
<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <start>
    <element name="root"/>
  </start>
</grammar>
XML
  validator = REXML::Validation::RelaxNG.new(rng)

  source = <<-XML
<?xml version="1.0" encoding="UTF-8"?>
<root/>
XML

  no_error(validator, source)
end

Start with the proposed test case in test/test_validation_rng.rb and run the RELAX NG validation tests. Confirm that an XML document containing the declaration validates successfully against the provided grammar; the implementation path is not specified in the issue.