Please describe the problem:
When posting messages to Japanese mailing list, it's important to use iso-2022-jp encoding since not all mailers support UTF-8 encoding.

Evolution allows users to manually specify the desired encoding via Edit - Character Encoding in the composer window, but choosing iso-2022-jp here only encodes the text body in iso-2022-jp while the subject line remains UTF-8 encoded.

I would expect the subject line to be encoded in the same encoding as the text body, which in this case is iso-2022-jp.

Steps to reproduce:
1. File - New - Mail Message to launch a composer window.
2. Go to Edit - Character Encoding and select iso-2022-jp
3. Write mail and send it out.  You can copy and paste the following for testing (the text between >- and <-)

Subject: ->日本語のテスト<-
Text body: >-テストメールです。<-

Actual results:
the Subject line is UTF-8 encoded.  You can see when you check the source of your email.  Here is what I get:

Subject: =?UTF-8?Q?=E6=97=A5=E6=9C=AC=E8=AA=9E=E3=81=AE=E3=83=86=E3=82=B9?=
        =?UTF-8?Q?=E3=83=88?=

Expected results:
I would expect the subject line to be iso-2022-jp encoded.

Does this happen every time?
Yes.

Other information:
This bug IMO is very important for Japanese-language users.  UTF-8 encoded emails are frowned upon in Japanese language mailing lists in general.  While some lists are receptive of UTF-8 encoded emails, others are very vocal about discriminating against them.