#18 Encoding auto get converted to Custom

[Pravin Satpute]

While changing character Unicode Value to "-1" Encoding of sfd file automatically get converted to Custom.

Steps 2 reproduce
1. Open sfd file
2. Set Encoding Unicode BMP
3. Now open any character and Set Unicode Value to -1
4. Encoding automatically get converted to custom.

This is happening on Fedora 17, fontforge-20110222-7.fc17.x86_64
with Liberation fonts fontforge-20110222-7.fc17.x86_64