Always enable limit for combined queries

[gluk-w]

# Let's say queryset consists of unions
qs = qs.union(...)
qs = qs.union(...)

# Let's say queryset returns 20 objects total
assert qs.count() == 20

# Works fine if offset is higher than zero because "OFFSET 1 ROWS FETCH FIRST 10 ROWS ONLY" is applied
assert len(list(qs[1:10])) == 10

# This fails without my fix because "OFFSET 0 ROWS FETCH FIRST 10 ROWS ONLY" is not applied
assert len(list(qs[0:10])) == 10