Apply patches from kuroda/iso-2022-jp

I am maintainer of the `mail-iso-2022-jp` gem, a set of patches against `mail` gem for `ISO-2022-JP` encoding.

I applied these patches on the `mail` source tree upon request from @mikel.

Note that these patches have effects only when a `Mail` object is instanciated with `:charset => 'ISO-2022-JP'` option. With this option, the header values and body text are *automatically* converted from `UTF-8` to `ISO-2022-JP`.

I am afraid that someone does not like this automatic conversion feature arguing that it is not scope of this library.

I submit this pull request as a start point of discussion. Please let me know if there is a room for improvement.