connection emits the exhausted event when the strem limit has been achieved on an HTTP/2 connection

this happens when the requests are enqueued before the connection is
negotiated, and then one has too many requests enqueued.

lib/httpx/connection.rb

@@ -292,6 +292,9 @@ module HTTPX
       parser.on(:promise) do |request, stream|
         request.emit(:promise, parser, stream)
       end
+      parser.on(:exhausted) do
+        emit(:exhausted)
+      end
       parser.on(:origin) do |origin|
         @origins << origin
       end

lib/httpx/connection/http2.rb

@@ -56,6 +56,9 @@ module HTTPX
       end
       handle(request, stream)
       true
+    rescue HTTP2Next::Error::StreamLimitExceeded
+      @pending.unshift(request)
+      emit(:exhausted)
     end
 
     def consume