Fix missing contextPath when mutating ServerHttpRequest

This commit ensures that when mutating `ServerHttpRequest` instances,
the original contextPath information is copied to the request being
built.

Note that mutation on the `contextPath(String)` or `path(String)` should
be reflected to the other. (See their Javadoc for more information).

Fixes gh-25279

spring-web/src/main/java/org/springframework/http/server/reactive/DefaultServerHttpRequestBuilder.java

@@ -71,6 +71,7 @@ class DefaultServerHttpRequestBuilder implements ServerHttpRequest.Builder {
 
 		this.uri = original.getURI();
 		this.httpMethodValue = original.getMethodValue();
+		this.contextPath = original.getPath().contextPath().value();
 		this.body = original.getBody();
 
 		this.httpHeaders = HttpHeaders.writableHttpHeaders(original.getHeaders());