bugfix unfolding email multi-line header 

The actual unfolding strategy is broken, see section "Long Header Fields" on RFC 2822
http://tools.ietf.org/html/rfc2822#section-2.2.3

Original E-Mail header:
```
Subject: Hello
 World
```

The actual unfolding:
```
Subject: HelloWorld
```

The expected unfolding:
```
Subject: Hello World
```