Crash fix: HTTP2 can handle requests are cancelled (#555)

Co-authored-by: George Barnett <gbarnett@apple.com>

Sources/AsyncHTTPClient/ConnectionPool/HTTP2/HTTP2ClientRequestHandler.swift

@@ -193,11 +193,15 @@ final class HTTP2ClientRequestHandler: ChannelDuplexHandler {
         case .forwardResponseBodyParts(let parts):
             self.request!.receiveResponseBodyParts(parts)
 
-        case .failRequest(let error, let finalAction):
+        case .failRequest(let error, _):
             self.request!.fail(error)
             self.request = nil
             self.runTimeoutAction(.clearIdleReadTimeoutTimer, context: context)
-            self.runFinalAction(finalAction, context: context)
+            // No matter the error reason, we must always make sure the h2 stream is closed. Only
+            // once the h2 stream is closed, it is released from the h2 multiplexer. The
+            // HTTPRequestStateMachine may signal finalAction: .none in the error case (as this is
+            // the right result for HTTP/1). In the h2 case we MUST always close.
+            self.runFinalAction(.close, context: context)
 
         case .succeedRequest(let finalAction, let finalParts):
             self.request!.succeedRequest(finalParts)