Fix #leaves used with a custom relation

[attilahorvath]

Currently #leaves doesn't work properly with chained custom relations.

The #where_values_hash method used in its implementation doesn't return every where condition, e.g. it ignores the ones passed as raw SQL fragment strings, leading to an empty hash which then causes the method to just return an empty array.

Here is a test case to illustrate the issue:

TreeNode.where("path <> 'Top.Collections.Pictures.Astronomy.Stars'")
        .leaves
        .pluck(:path) #=> []

This PR fixes the issue by rewriting the query to check for leaf nodes based on https://stackoverflow.com/a/43512352.

[sjke]

@attilahorvath thx for your fix.