Fix g_mime_content_encoding_to_string return null for …DEFAULT value

[mjog]

Its API docs say that GMIME_CONTENT_ENCODING_DEFAULT is a valid value,
but that is missed in the switch, resulting in null being returned
when that is passed in.

This also fixes expected behaviour for the Vala bindings, where a
string is returned for any valid enum value.

[jstedfast]

So the way this is meant to be handled is if NULL is returned, you just don't add a Content-Transfer-Encoding header. Otherwise the value returned should be set as the value. A value of "default" isn't helpful because there is no such encoding and no code in the wild exists that will check for it.