Add some tests for Literal#english?

spec/model_literal_spec.rb

@@ -133,6 +133,22 @@ def self.literals(*selector)
     end
   end
 
+  describe "#english?" do
+    literals(:all).each do |args|
+      options = args.last.is_a?(Hash) ? args.pop : {}
+      lit = RDF::Literal.new(*args, **options)
+      if lit.language? && lit.language.to_s.downcase.start_with?('en')
+        it "returns true for #{lit.inspect}" do
+          expect(lit).to be_english
+        end
+      else
+        it "returns false for #{lit.inspect}" do
+          expect(lit).not_to be_english
+        end
+      end
+    end
+  end
+
   describe "#datatype" do
     literals(:all_simple).each do |args|
       it "returns xsd:string for #{args.inspect}" do