bsearch return unexpected value when using less or equal test

[SlimBenAmor]

As described in the comment of [bsearch_index](https://github.com/crystal-lang/crystal,

[2, 5, 7, 10].bsearch_index { |x, i| x >= 4 } # => 1
[2, 5, 7, 10].bsearch_index { |x, i| x > 10 } # => nil

which is correct. However, if we test with the following, we get unexpected result.

[2, 5, 7, 10].bsearch_index { |x| x <= 5}} # => nil
[2, 5, 7, 10].bsearch_index { |x| x <= 7} # => 0

I think there is a bug. And the problem seems to appear if the searched value in in the first half of the array (we will always get nil) or in the second half (we will always get 0).

[spuun]

It works as intended. If you read the docs for the method it mentions that "binary search needs the collection to be sorted in regards to the search criterion". I.e if you have a list sorted ascending you must use a greater than comparison, while if the list is sorted descending you must use a less than comparison.