FIX support for nested arrays with config.validate_keys = true

[misdoro]

Before this change, it was impossible to validate schema for nested arrays of strings or hashes in params, when config.validate_keys = true was set.

Before:

schema = Dry::Schema.Params do
  config.validate_keys = true
  required(:matrix).value(:array, size?: 2).each do
    value(:array, size?: 3).each do
      value(:string, size?: 1)
    end
  end
end

errors = schema.call(matrix: [["a", "b", "c"], ["d", "ee", "f"]]).errors

puts errors.to_h.inspect

would fail validation saying:

{matrix: {0 => {a: ["is not allowed"], b: ["is not allowed"], c: ["is not allowed"]}, 1 => {d: ["is not allowed"], ee: ["is not allowed"], f: ["is not allowed"]}}}

After

a correct validation error for the string length is reported

{:matrix=>{1=>{1=>["length must be 1"]}}}

[misdoro]

Hi @timriley @radar may I ask you for a review please?

[timriley]

Thanks @misdoro, this looks like a well put together change :) I'll review it as soon as I can. Sometime in the next ~week.