Custom ransackers not following case-insensitive behavior

[RaymondFallon]

When defining a custom ransacker and using it in combination with a case-insensitive predicate, the case-insensitivity is not being correctly applied (on SQLite and other adapters lacking the ILIKE operator).

For an example, see this Gist demonstrating case insensitivity being applied correctly to a default ransacker, first_name, but not being applied to a custom ransacker, full_name:

https://gist.github.com/RaymondFallon/aa49b5c5e317f208c0ac5e527ea2bf77