Handle correctly nested declared params

Hi,

the method `declared` from `Grape::Endpoint` wasn't handling correctly the case when there were grouped params

eg:
```
require 'grape'

class Test < Grape::API
  params do
    group :nested do
      requires :value
    end
  end
  get "/" do
    declared(params).to_json
  end
end

run Test
```

`curl localhost:9292/\?nested\[value\]\=3`

currently this returns `{:value=>nil}` instead of `{:nested => { :value => 3 } }`

Please let me know if you want me to change or add something

Thanks!!!