Don't check for valid list unless subject is a BNode.

This avoids DEPRECATION warning, and addresses issue #30.

Author: gkellogg

lib/rdf/rdfxml/writer.rb

@@ -230,6 +230,7 @@ def render_property(predicate, objects, options = {}, &block)
 
       # Separate out the objects which are lists and render separately
       lists = objects.
+        select(&:node?).
         map {|o| RDF::List.new(o, @graph)}.
         select {|l| l.valid? && l.none?(&:literal?)}
 

spec/writer_spec.rb

@@ -111,7 +111,7 @@ class FOO < RDF::Vocabulary("http://foo/"); end
       end
     end
 
-    context "with children", focus: true do
+    context "with children" do
       subject do
         @graph << [RDF::URI.new("http://release/"), RDF.type, FOO.Release]
         @graph << [RDF::URI.new("http://release/"), RDF::DC.title, "foo"]