Hi, and thanks for the great projet.

I made this first contribution because it feels like the order is wrong here: If a hash or array is given, the set(k,v) line would never be taken into account...

In fact I came accross this issue because using both sinatra/namespace and sinatra-cross_origin doesn't work for local cross origin. Reproduction be like:

namespace "/foo" do
  get  "/bar" do
    cross_origin allow_origin: "example.com"
    send_file "bar"
  end
end

However, I do think that by design you want to block any global keys to be set from a namespace and do not have enough understanding of how this may work. Hence I'm thinking about fixing this issue directly in the sinatra-cross_origin gem. Any input on this one would be much appreciated of course :)