Implement hash equality on column

The query cache uses bind values as hash keys. The current
implementation relies on reference equality for hash equality. This is
brittle, and can easily break in the future.

activerecord/lib/active_record/connection_adapters/column.rb

@@ -64,6 +64,11 @@ def ==(other)
           other.sql_type == sql_type &&
           other.null == null
       end
+      alias :eql? :==
+
+      def hash
+        [self.class, name, default, cast_type, sql_type, null].hash
+      end
     end
   end
   # :startdoc: