Connection upgrade fails when request body is not empty

[noteflakes]

Hi and thanks for this wonderful gem! When upgrading an HTTP/1.1 connection (e.g. using curl --http2), if the HTTP/1.1 request body passed to HTTP2::Server#upgrade is not empty, the upgrade will fail with the following traceback:

Traceback (most recent call last):
        12: from /home/sharon/repo/tipi/lib/tipi.rb:39:in `block (2 levels) in accept_loop'
<snip>
         3: from /home/sharon/.gem/gems/http-2-0.11.0/lib/http/2/server.rb:102:in `upgrade'
         2: from /home/sharon/.gem/gems/http-2-0.11.0/lib/http/2/stream.rb:102:in `receive'
         1: from /home/sharon/.gem/gems/http-2-0.11.0/lib/http/2/stream.rb:321:in `transition'
/home/sharon/.gem/gems/http-2-0.11.0/lib/http/2/stream.rb:604:in `end_stream?': undefined method `include?' for nil:NilClass (NoMethodError)

The fix is trivial:

class HTTP2::Stream
  def end_stream?(frame)
    case frame[:type]
    when :data, :headers, :continuation
      frame[:flags]&.include?(:end_stream)
    else false
    end
  end
end

The fix is just to add the safe navigation operator when checking for frame[:flags], since it's its not set when the body is not empty.