ClosureExpressionVisitor::sortByField() decided ties with a strict
"===" identity check before falling through to the next sort field.
For object-typed fields (e.g. two DateTimeImmutable instances that
represent the same point in time but are not the same instance),
this check never passes, so secondary/tertiary Criteria::orderBy()
fields were silently ignored and the resulting comparator was not
antisymmetric (swapping the input order of two such elements changed
the sort result).
Replace the "===" tie check with the same "<=>" comparison already
used to determine ordering, so a tie is now detected whenever the
values are equal under that comparison, matching value semantics for
both scalars and comparable objects.
Fixes#437
