Retain Literal equality when setting a relation

Fixes the bug in #245 causing literal languges to be lost when setting
values to an existing `Relation` (including self).

lib/active_triples/relation.rb

@@ -137,7 +137,8 @@ def <=>(other)
     #
     # @return [Relation] a relation containing the set values; i.e. `self`
     def <<(values)
-      values = to_a | Array.wrap(values)
+      values = values.objects.to_a if values.is_a?(Relation)
+      values = objects.to_a | Array.wrap(values)
       self.set(values)
     end
     alias_method :push, :<<
@@ -380,7 +381,7 @@ def property
     #   non-{RDF::Resource} values.
     def set(values)
       raise UndefinedPropertyError.new(property, reflections) if predicate.nil?
-      values = values.to_a if values.is_a? Relation
+      values = values.objects.to_a if values.is_a? Relation
       values = [values].compact unless values.kind_of?(Array)
 
       clear

spec/active_triples/relation_spec.rb

@@ -473,6 +473,38 @@ class WithTitle
       expect { subject << values }
         .to change { subject.to_a }.to contain_exactly(*values)
     end
+
+    it 'keeps datatypes' do
+      values = [RDF::Literal(Date.today), RDF::Literal(:moomin)]
+
+      expect { values.each { |v| subject << v } }
+        .to change { subject.send(:objects).to_a }
+             .to contain_exactly(*values)
+    end
+
+    it 'keeps languages' do
+      values = [RDF::Literal("Moomin", language: :en), 
+                RDF::Literal("Mummi",  language: :fi)]
+
+      expect { values.each { |v| subject << v } }
+        .to change { subject.send(:objects).to_a }
+             .to contain_exactly(*values)
+    end
+
+    context 'when given a Relation' do
+      it 'keeps datatypes and languages of values' do
+        values = [RDF::Literal(Date.today),
+                  RDF::Literal(:moomin), 
+                  RDF::Literal("Moomin", language: :en),
+                  RDF::Literal("Mummi",  language: :fi)]
+
+        subject.set(values)
+        expect(subject.send(:objects)).to contain_exactly(*values)
+
+        expect { subject << subject }
+          .not_to change { subject.send(:objects).to_a }
+      end
+    end
   end
 
   describe '#predicate' do
@@ -714,6 +746,21 @@ class WithTitle
           .to change { subject.to_a }.to contain_exactly(*values)
       end
 
+      context 'when given a Relation' do
+        it 'keeps datatypes and languages of values' do
+          values = [RDF::Literal(Date.today),
+                    RDF::Literal(:moomin), 
+                    RDF::Literal("Moomin", language: :en),
+                    RDF::Literal("Mummi",  language: :fi)]
+
+          subject.set(values)
+          expect(subject.send(:objects)).to contain_exactly(*values)
+
+          expect { subject.set(subject) }
+            .not_to change { subject.send(:objects).to_a }
+        end
+      end
+
       context 'and persistence config' do
         before do
           reflections