Gateway should handle header 'Expect 100-continue' compliant with rfc2616

• If a proxy receives a request that includes an Expect request-
  header field with the "100-continue" expectation, and the proxy
  either knows that the next-hop server complies with HTTP/1.1 or
  higher, or does not know the HTTP version of the next-hop
  server, it MUST forward the request, including the Expect header
  field.

Steps to reproduce problem

Target server: Spring Boot 2.1.8 (Tomcat)

@SpringBootApplication @RestController 
public class MyDownstreamApp {
   public static void main(String[] args) {
      SpringApplication.run(MyDownstreamApp.class, args);
  }

  @PutMapping("/put")
  String put(@RequestParam String x) {
      System.out.println("x: " +x);
      return "hello";
  }
}

Cloud gateway: Greenwich SR3

@SpringBootApplication
public class MyGatewayApp {

    @Bean
    RouteLocator myRoutes(RouteLocatorBuilder builder) {
        return builder.routes()
                .route(p -> p
                        .path("/put")
                        .uri("http://localhost:8080"))
                .build();
    }

    public static void main(String[] args) {
        SpringApplication.run(MyGatewayApp.class, "--server.port=8082");
    }
}

When target server is called directly:
curl localhost:8080/put -X PUT -H 'Expect: 100-continue' -d x=foo1 -v

communication flow is OK

1. Client sends Expect: 100-continue
2. Server reponds 100-continue
3. Client sends complete request
4. Server responds hello

> Expect: 100-continue
> Content-Length: 6
> Content-Type: application/x-www-form-urlencoded
>
< HTTP/1.1 100
* We are completely uploaded and fine
< HTTP/1.1 200
< Content-Type: text/plain;charset=UTF-8
< Content-Length: 5

When target server is called through gateway:
curl localhost:8082/put -X PUT -H 'Expect: 100-continue' -d x=foo1 -v

communication flow is not OK. My wireshark investigation

1. Client sends Expect: 100-continue to gateway
2. Gateway responds 100-continue
3. Client sends complete request to gateway
4. I'm not sure 100% what happens in point 4
5. Target server responds 100-continue to gateway
6. Gateway sends complete request to target server
7. Target server responds hello
8. Gateway forwards 100-continue to client
9. Client waits for response hello but it is not forwarded

> Expect: 100-continue
> Content-Length: 6
> Content-Type: application/x-www-form-urlencoded
>
< HTTP/1.1 100 Continue
* We are completely uploaded and fine
< HTTP/1.1 100 Continue
(hang up)

Response 100 Continue is received 2 times - once generated by gateway itself, second forwarded from target server, but true response never reach client.

Worth to notice that workaround is filter our header 'Expect 100-continue` when gateway forwards request to target server

Reproduce the behavior with the provided Spring Boot gateway and target-server examples, using the two curl commands and verbose output. Trace the Expect: 100-continue exchange between gateway and target; done means the request is forwarded compliantly without a duplicate interim response and the target's final hello response reaches the client.