The search results should be consistent between query 10 and queries 10..10.

In current implementation, the result is not consistent:
```rb
IntervalTree::Tree.new(10..10).search(10)
 => [10...11]

IntervalTree::Tree.new(10..10).search(10..10)
 => []
```

The result of query 10..10 should be `[10...11]`, too.
