close http2 connection as soon as it served all possible requests; this will make the persistent plugin account for resources better

lib/httpx/connection/http1.rb

@@ -47,7 +47,7 @@ module HTTPX
     end
 
     def send(request)
-      if !@max_requests.positive?
+      unless @max_requests.positive?
         @pending << request
         return
       end

lib/httpx/connection/http2.rb

@@ -202,6 +202,10 @@ module HTTPX
 
       @streams.delete(request)
       send(@pending.shift) unless @pending.empty?
+      return unless @streams.empty? && exhausted?
+
+      close
+      emit(:close)
     end
 
     def on_frame(bytes)