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. '[中文]'.
*) 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 "中文" (both mlmmj subject 
prefix and original mail subject are same Chinese word):

```
Subject: [中文] =?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.