Use `headers` method instead of `header` method in `Sinatra::Response`

Since `headers` is an alias for `header` (see `Rack::Response`) and `headers` is widely used in Sinatra. It's confusing that in `Sinatra::Response#finish` both `headers` and `header` are used.

And in `Sinatra::ExtendedRack#setup_close` use `headers` as parameter just like in `Sinatra::ExtendedRack#async?`