Refactor ServerWebExchange/ServerHttpRequest builders

ServerWebExchange.Builder has an additional Consumer-style shortcut
method that accepts a builder for modifying the request.

ServerWebExchange and ServerHttpRequest builders have fewer methods,
more use-case focused vs matching directly to properties.

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

@@ -18,7 +18,6 @@ package org.springframework.http.server.reactive;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
-import java.util.LinkedHashMap;
 import java.util.Map;
 
 import org.junit.Test;