Capture Standard output in writer tests.

spec/writer_spec.rb

@@ -8,7 +8,7 @@ class FOO < RDF::Vocabulary("http://foo/"); end
 describe "RDF::RDFXML::Writer" do
   let(:logger) {RDF::Spec.logger}
   it_behaves_like 'an RDF::Writer' do
-    let(:writer) {RDF::RDFXML::Writer.new}
+    let(:writer) {RDF::RDFXML::Writer.new(::StringIO.new)}
   end
 
   describe "#buffer" do