fix: handle multi goaway frames coming from server

nodejs servers, for example, seem to send them when shutting down servers on timeout; when receiving, in the same buffer, the first correctly closes the parser and emits the message, while the second, because the parser is closed already, will emit an exception; the regression happened because the second exception was swallowed by the pool handler, but now that's gone, and errors on connection consumption get handled; this was worked around by, on the parser, when emitting the errors for pending requests, claearing the queue, as when the second error comes, there's no request to emit the error for

Closes #333

lib/httpx/connection/http2.rb

@@ -137,7 +137,7 @@ module HTTPX
 
         emit(:error, req, ex)
       end
-      @pending.each do |req|
+      while (req = @pending.shift)
         next if request && request == req
 
         emit(:error, req, ex)