Ensure CORS processing does not add Vary header twice on async requests

[blackstorm]

spring boot version 2.2.2

example

WebMvcConfigurer

CorsRegistration corsRegistration = registry.addMapping("/**")
                .allowCredentials(true);
corsRegistration.allowedHeaders("*")
                .maxAge(1800L)
                .allowedMethods(HttpMethod.GET.name(),
                        HttpMethod.POST.name(),
                        HttpMethod.PUT.name(),
                        HttpMethod.OPTIONS.name(),
                        HttpMethod.DELETE.name())
                .exposedHeaders("Content-Length");

controller

@RequestMapping("/test)
@RestController
public class ExampleController {
    @GetMapping
    public CompletableFuture<T> test() {
        // do something
    }
}

reponse headers

Vary | Origin
Vary | Access-Control-Request-Method
Vary | Access-Control-Request-Headers
Vary | Origin
Vary | Access-Control-Request-Method
Vary | Access-Control-Request-Headers

Ref #24222

[pivotal-issuemaster]

@blackstorm Please sign the Contributor License Agreement!

Click here to manually synchronize the status of this Pull Request.

See the FAQ for frequently asked questions.

[pivotal-issuemaster]

@blackstorm Thank you for signing the Contributor License Agreement!