If there are hints, the aFormatSet.Differentiate(rSet) will cause the
pre-existing items in the parameter rSet to win, whereas without hints
the SwContentNode::GetAttr() calls rSet.Set() which clears everything.

It looks like WW8Export::WriteFootnoteBegin() was sort of relying on
this; presumably it wants the pCFormat to win over node's items but
hints on the position to win over pCFormat.  Well that's what it
currently gets anyway, so try to preserve that.

The other use in SwUnoCursorHelper::SetPropertyValues() looks like an
oversight.

Change-Id: I80e18e040413fcf26f6f77844ed731de31a464d2