add an operator to inequality

lib/arel/algebra/predicates.rb

@@ -197,6 +197,7 @@ def complement
         Equality.new(operand1, operand2)
       end
 
+      def operator; :"!=" end
       def eval(row)
         operand1.eval(row) != operand2.eval(row)
       end

spec/algebra/unit/predicates/inequality_spec.rb

@@ -12,6 +12,12 @@ module Predicates
         @b        = Inequality.new(right, left)
       end
 
+      describe 'operator' do
+        it "should have one" do
+          @a.operator.should == :"!="
+        end
+      end
+
       describe '==' do
         it "is equal to itself" do
           @a.should == @a