[mew-int 3134] preserving the original charset of a attach file

[Kazu Yamamoto ( 山本和彦 )]

Hello,

I have implemented a feature to preserve the original charset of a
attach file. Pulling from github and setting

	(setq mew-draft-keep-text-charset t)

enables this feature. 

For instance, UTF-8 files are always attached as UTF-8 files if 
this feature is enabled. Only line delimiters are converted.

--Kazu