Make utf8::encode respect magic

Make utf8::encode respect magic

It has always ignored set-magic, as far as I can tell.

Since the magic flags patch (4bac9ae47b), it has been ignor-
ing get- magic on magical scalars that were already PVs.
sv_utf8_upgrade_flags_grow begins with an if(!SvPOK(sv)) check, which
used to mean ‘if this scalar is magic or not a string’, but now means
simply ‘if this scalar is not a string’.  SvPOK_nog is the new SvPOK.

Due to the way the flags now work, I had to modify sv_pvutf8n_force
as well, to keep existing tests passing.