[perl #101738] Make sv_sethek set the UTF8 flag correctly

[perl #101738] Make sv_sethek set the UTF8 flag correctly

It was only ever turning it on, and not turning it off if the sv hap-
pened to have it on from its previous use.

This caused ref() (which uses sv_sethek(TARG,...)) to return a shared
scalar with the UTF8 flag on, even if it was supposed to be off.

For shared scalars, the UTF8 flag on ASCII strings does make a differ-
ence.  The pv *and* the flags are used in hash lookup, for speed.

So a scalar returned by ref() with the UTF8 flag on by mistake would
not work in hash lookups.  exists $classes{ref $foo} would return
false, even if there were an entry for that class.