Tolerate multiple calls to shutdown()

Author: dnadoba

Sources/AsyncHTTPClient/ConnectionPool/HTTP2/HTTP2Connection.swift

@@ -279,10 +279,11 @@ final class HTTP2Connection {
             // are closed.
             self.channel.triggerUserOutboundEvent(HTTPConnectionEvent.shutdownRequested, promise: nil)
 
-        case .closed:
-            // we are already closed and we need to tolerate this
+        case .closed, .closing:
+            // we are already closing/closed and we need to tolerate this
             break
-        case .initialized, .closing:
+            
+        case .initialized:
             preconditionFailure("invalid state \(self.state)")
         }
     }