Generate more sqlish queue.

Now, instead of the following SQL code:
  some_field IN (1, 2, NULL)

Arel will generate the proper one:
  some_field IN (1, 2) OR IS NULL

lib/arel/predications.rb

@@ -41,7 +41,17 @@ def in other
           Nodes::Between.new(self, Nodes::And.new([other.begin, other.end]))
         end
       else
-        Nodes::In.new self, other
+        if other.include?(nil)
+          if other.size > 1
+            set  = Nodes::In.new self, other.compact
+            null = Nodes::Equality.new self, nil
+            Nodes::Or.new set, null
+          else
+            Nodes::Equality.new self, nil
+          end
+        else
+          Nodes::In.new self, other
+        end
       end
     end