Description:
------------
I use a Chinese word as mail subject prefix, but mlmmj doesn't encode it.
For example:
*) in mlmmj, mail subject prefix is set to a Chinese word, e.g. '[ä¸æ–‡]'.
*) Compose a message with subject: "Test"
*) Email sent out by mlmmj has this mail header:
```
Subject: <Chinese> Test
```
Another real-world example, compose email with subject "ä¸æ–‡" (both mlmmj subject
prefix and original mail subject are same Chinese word):
```
Subject: [ä¸æ–‡] =?UTF-8?Q?=E4=B8=AD=E6=96=87?=
```
Expected result:
----------------
Mlmmj should construct new mail subject with mlmmj subject prefix and original mail
subject first, then encode it.