custom validator keeping ref to instance variable

[loicginoux]

This is a failing test that demonstrate a possible bug. This is a failing test in response to this issue #1592.

I have tested both from code at version 0.16.2 and master with similar error.

Not sure if the issue is in custom validator or in the helper. I hope someone knowing better the source code can help on this. Thanks

[grape-bot]

	1 Warning
:warning:	Unless you’re refactoring existing code, please update CHANGELOG.md.

	1 Message
:book:	We really appreciate pull requests that demonstrate issues, even without a fix. That said, the next step is to try and fix the failing tests!

Here's an example of a CHANGELOG.md entry:

* [#1593](https://github.com/ruby-grape/grape/pull/1593): Custom validator keeping ref to instance variable - [@loicginoux](https://github.com/loicginoux).

Generated by :no_entry_sign: danger

[dnesteryuk]

@dblock This one might be closed, the related issue was fixed.

[dblock]

@loicginoux Want to rebase and retest? Close or update if the test case is useful and I'll merge.