Params: optional arrays within hashes/JSON objects broken

[dleavitt]

See failing tests below for more specifics.

To reproduce
In a params block, define an array param within a Hash or JSON param:

params do
  optional :data, type: Hash do
    optional :array, type: Array do
      optional :name, type: String
    end
  end
end

When the request body looks like this: { data: {} }
I'd expect that calling declared(params) within the route would return something like this:
{ data: { array: [] } }
or maybe this:
{ data: {} }

What's actually returned is this:
{ data: { array: { name: nil } } }

A hash rather than an array.

[grape-bot]

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

	1 Message
📖	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:

* [#2001](https://github.com/ruby-grape/grape/pull/2001): Params: optional arrays within hashes/json objects broken - [@dleavitt](https://github.com/dleavitt).

Generated by 🚫 danger

[myxoh]

Makes sense, thanks for the test!

[dblock]

@dleavitt now that we have a test (thanks), try fixing it?

[dnesteryuk]

I guess here we should expect the hash. @dleavitt could you verify this test?