Add failing test for multiple nested params error indices

[infiniteluke]

This reproduces the issue described in #1998. The AttributesIterator seems to control what the a given scopes index is set to. I believe this bug is due to index not iterating in sync with each param being validating in an array.

You can see here that the scope is set to 2 before we even start validating the value param:

[grape-bot]

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

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

* [#2055](https://github.com/ruby-grape/grape/pull/2055): Add failing test for multiple nested params error indices - [@infiniteluke](https://github.com/infiniteluke).

Generated by :no_entry_sign: danger

[StefanHagen]

Hey @infiniteluke ! This is exactly the issue I'm having currently, and I see the issue is still open. Did you find a workaround for this problem? Thanks for making that spec!