Perl: fixed $r->print() zero copy with mutable scalars

Perl: fixed $r->print() zero copy with mutable scalars

Since d9b7669666c8, the single-SV zero copy path in $r->print() no
longer required the SV to be read-only.  Reference counting introduced
by that change keeps the SV alive, but not its string buffer: modifying
a mutable scalar after $r->print() overwrites the buffer in place or
reallocates it, while nginx might still have it queued in r->out, since
small responses are postponed by ngx_http_write_filter().  This resulted
in corrupted responses, and in freed memory being sent to the client
when the scalar was grown.

The SV is now required to be read-only again.  Reference counting is
still needed here, as a read-only SV in an eval() might be freed with
the eval's op tree.