Don’t sv_force_normal in mg.c:S_save_magic

Don’t sv_force_normal in mg.c:S_save_magic

This was added to make SvREADONLY_off safe.  (I think read-only is
turned off during magic so the magic scalar itself can be set without
the sv_set* functions getting upset.)  Since SvREADONLY doesn’t mean
read-only for COWs, we don’t actually need to do sv_force_normal, but
can simply skip SvREADONLY_off for COWs.

By leaving it to sv_set* functions to do sv_force_normal, we avoid
having to copy the string buffer if it is just going to be thrown away
anyway.  S_save_magic can’t know whether the scalar will actually be
overwritten, so it has to copy the buffer.