Better support for String primary keys

Belongs to #60

lib/unread/readable_scopes.rb

@@ -7,7 +7,7 @@ def join_read_marks(user)
         joins "LEFT JOIN read_marks
                 ON read_marks.readable_type  = '#{base_class.name}'
                AND read_marks.readable_id    = #{quoted_table_name}.#{quoted_primary_key}
-               AND read_marks.user_id        = #{user.id}
+               AND read_marks.user_id        = #{quote_bound_value(user.id)}
                AND read_marks.timestamp     >= #{quoted_table_name}.#{connection.quote_column_name(readable_options[:on])}"
       end
 
@@ -40,7 +40,7 @@ def read_by(user)
       def with_read_marks_for(user)
         join_read_marks(user).select("#{quoted_table_name}.*,
                                      read_marks.id AS read_mark_id,
-                                     #{user.id} AS read_mark_user_id")
+                                     #{quote_bound_value user.id} AS read_mark_user_id")
       end
     end
   end