Polish AbstractServerHttpResponse

This commit simplifies the logic for applying beforeCommit actions
replacing the use of chained Mono.then calls with a single
Flux.concat.

Also renamed writeStatusCode, writeHeaders, and writeCookies to
applyStatusCode, applyHeaders, and applyCookies respectively to
better reflect we're simply setting them on the underlying response
(not necessarily written yet).

spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpResponseTests.java

@@ -158,19 +158,19 @@ public class ServerHttpResponseTests {
 		}
 
 		@Override
-		public void writeStatusCode() {
+		public void applyStatusCode() {
 			assertFalse(this.statusCodeWritten);
 			this.statusCodeWritten = true;
 		}
 
 		@Override
-		protected void writeHeaders() {
+		protected void applyHeaders() {
 			assertFalse(this.headersWritten);
 			this.headersWritten = true;
 		}
 
 		@Override
-		protected void writeCookies() {
+		protected void applyCookies() {
 			assertFalse(this.cookiesWritten);
 			this.cookiesWritten = true;
 		}