Dedupe response header filter (#866)

Filter that de-duplicates response filter values. Comes handy when e.g. both the gateway CORS logic and a downstream add Access-Control-Allow-Credentials and Access-Control-Allow-Origin headers. See https://github.com/spring-cloud/spring-cloud-gateway/issues/728 for additional context.

docs/src/main/asciidoc/spring-cloud-gateway.adoc

@@ -369,7 +369,7 @@ spring:
   cloud:
     gateway:
       routes:
-      - id: add_request_header_route
+      - id: add_response_header_route
         uri: http://example.org
         filters:
         - AddResponseHeader=X-Response-Foo, Bar
@@ -377,6 +377,26 @@ spring:
 
 This will add `X-Response-Foo:Bar` header to the downstream response's headers for all matching requests.
 
+=== DedupeResponseHeader GatewayFilter Factory
+The DedupeResponseHeader GatewayFilter Factory takes a `name` parameter and an optional `strategy` parameter. `name` can contain a list of header names, space separated. 
+
+.application.yml
+[source,yaml]
+----
+spring:
+  cloud:
+    gateway:
+      routes:
+      - id: dedupe_response_header_route
+        uri: http://example.org
+        filters:
+        - DedupeResponseHeader=Access-Control-Allow-Credentials Access-Control-Allow-Origin
+----
+
+This will remove duplicate values of `Access-Control-Allow-Credentials` and `Access-Control-Allow-Origin` response headers in cases when both the gateway CORS logic and the downstream add them. 
+
+The DedupeResponseHeader filter also accepts an optional `strategy` parameter. The accepted values are `RETAIN_FIRST` (default), `RETAIN_LAST`, and `RETAIN_UNIQUE`.
+
 === Hystrix GatewayFilter Factory
 https://github.com/Netflix/Hystrix[Hystrix] is a library from Netflix that implements the https://martinfowler.com/bliki/CircuitBreaker.html[circuit breaker pattern].
 The Hystrix GatewayFilter allows you to introduce circuit breakers to your gateway routes, protecting your services from cascading failures and allowing you to provide fallback responses in the event of downstream failures.