Passing an array of string to params expecting an array of hash raises NoMethodError

This is a bug report. I can probably fix it, but wanted to create an issue first.

In my API, I have declared something like the following:

# class
class Foo < Grape::API
  desc "Do foo"
  params do
    requires :foo, type: Array do
      optional :bar # Expects { "foo": [{ "bar": "baz" }] }
    end
  end
  post do
    puts declared(params)
  end
end

# specs
RSpec.describe Foo do
  it 'does do foo' do
    params = { foo: [""] }
    post "/", params, {} do
      # raises 500 error
    end
  end
end

It is raising an error because of https://github.com/ruby-grape/grape/blob/master/lib/grape/dsl/inside_route.rb#L39-L50. In line 41, params || {} is supposed to generate a hash, but for non-falsy value, it raises NoMethodError key? because a string does not respond to key?.

Start with lib/grape/dsl/inside_route.rb at lines 39-50, then reproduce the reported request using the Foo API and RSpec example in the issue. Trace how an array containing a string is handled by the parameter declaration; done means the request no longer raises NoMethodError and the resulting behavior is covered by a regression test.