Add ignore_content_type option to json and form params

[vladfaust]

  params do
    json(ignore_content_type: true) do
      type content : String
    end
  end

Will attempt to parse the JSON body regardless of the "Content-Type" header, but only the body is present. Would raise 400 on body type mismatch (i.e. when form is passed). Would raise 400 if body is absent and this param type is required (see #59).

It would be useful for single-content-type endpoints.