Backport !2934 “gobject: Always ref-sink variants in g_object_set” to glib-2-74

When collecting varargs, ignore the NOCOPY_CONTENTS flag for variants. That is what our docs advice for refcounted types, and it fixes a regression that was inadvertendly introduced when we stopped doing some extra GValue copies.

Includes a test case by Philip Withnall.

Fixes: #2774 (closed)

---

Trivial backport of !2934 (merged) to glib-2-74.