Array group parameters

[adie]

Hi, I have an issue that I believe was fixed by #221 but then got overridden by #455.
I think it was needed to support both cases, not just "fix" those specs: https://github.com/ruby-grape/grape-swagger/pull/455/files#diff-2b857d0b8829328970ec856ccd7d8ac3L43

Right now things like

requires :group, type: Array do
  requires :param_1, type: Integer
  requires :param_2, type: String
end

are being displayed as a simple Hash, not as an Array.

I would expect to have group[][param_1] and group[][param_2] instead