Make update_all preparable

Before:

```
  Pet Update All (0.8ms)  UPDATE `pets` LEFT OUTER JOIN `toys` ON `toys`.`pet_id` = `pets`.`pet_id` SET `pets`.`name` = 'Bob' WHERE `toys`.`name` = ?  [["name", "Bone"]]
```

After:

```
  Pet Update All (1.1ms)  UPDATE `pets` LEFT OUTER JOIN `toys` ON `toys`.`pet_id` = `pets`.`pet_id` SET `pets`.`name` = ? WHERE `toys`.`name` = ?  [["name", "Bob"], ["name", "Bone"]]
```

activerecord/lib/active_record/relation.rb

@@ -348,10 +348,14 @@ module ActiveRecord
       end
 
       stmt = Arel::UpdateManager.new
-
-      stmt.set Arel.sql(@klass.sanitize_sql_for_assignment(updates, table.name))
       stmt.table(table)
 
+      if updates.is_a?(Hash)
+        stmt.set _substitute_values(updates)
+      else
+        stmt.set Arel.sql(klass.sanitize_sql_for_assignment(updates, table.name))
+      end
+
       if has_join_values? || offset_value
         @klass.connection.join_to_update(stmt, arel, arel_attribute(primary_key))
       else
@@ -625,6 +629,14 @@ module ActiveRecord
       end
 
     private
+      def _substitute_values(values)
+        values.map do |name, value|
+          attr = arel_attribute(name)
+          type = klass.type_for_attribute(attr.name)
+          bind = predicate_builder.build_bind_attribute(attr.name, type.cast(value))
+          [attr, bind]
+        end
+      end
 
       def has_join_values?
         joins_values.any? || left_outer_joins_values.any?

activerecord/lib/arel/visitors/mysql.rb

@@ -37,6 +37,10 @@ module Arel # :nodoc: all
           visit o.expr, collector
         end
 
+        def visit_Arel_Nodes_UnqualifiedColumn(o, collector)
+          visit o.expr, collector
+        end
+
         ###
         # :'(
         # http://dev.mysql.com/doc/refman/5.0/en/select.html#id3482214