Isolate Transactable#transaction test from blank nodes

This test used to implictly test stablity of blank node identifers
across multiple `#statements` calls. By using URIs instead of blank
nodes, we avoid failures when they change.

lib/rdf/spec/transactable.rb

@@ -18,7 +18,9 @@
     end
 
     it 'commits a successful transaction' do
-      statement = RDF::Statement(:s, RDF.type, :o)
+      statement = RDF::Statement(RDF::URI('http://example.com/s'), 
+                                 RDF.type, 
+                                 RDF::URI('http://example.com/o'))
       expect(subject).to receive(:commit_transaction).and_call_original
 
       expect do