Throwing an `:error` inside a middleware doesn't respect the `format` settings.

[dblock]

      it 'raised :error from middleware' do
        middleware = Class.new(Grape::Middleware::Base) do
          def before
            throw :error, message: "Unauthorized", status: 42
          end
        end
        subject.format :xml
        subject.use middleware
        subject.get do

        end
        get "/"
        last_response.status.should == 42
        last_response.body.should == <<-XML
<?xml version="1.0" encoding="UTF-8"?>
<error>
  <message>Unauthorized</message>
</error>
XML
      end

The format selected is :txt.