Avoid copying in DefaultServerHttpRequestBuilder

This commit avoids copying HTTP headers when mutating an HTTP request.
Instead, we're now unwrapping the `ReadOnlyHttpHeaders` (which is most
likely backed by the native request headers).

Issue: SPR-17250

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -1617,4 +1617,17 @@ public static HttpHeaders readOnlyHttpHeaders(HttpHeaders headers) {
 		}
 	}
 
+	/**
+	 * Return a {@code HttpHeaders} object that can read and written to.
+	 */
+	public static HttpHeaders writableHttpHeaders(HttpHeaders headers) {
+		Assert.notNull(headers, "HttpHeaders must not be null");
+		if (headers instanceof ReadOnlyHttpHeaders) {
+			return new HttpHeaders(headers.headers);
+		}
+		else {
+			return headers;
+		}
+	}
+
 }

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

@@ -72,8 +72,7 @@ public DefaultServerHttpRequestBuilder(ServerHttpRequest original) {
 		this.httpMethodValue = original.getMethodValue();
 		this.body = original.getBody();
 
-		this.httpHeaders = new HttpHeaders();
-		copyMultiValueMap(original.getHeaders(), this.httpHeaders);
+		this.httpHeaders = HttpHeaders.writableHttpHeaders(original.getHeaders());
 
 		this.cookies = new LinkedMultiValueMap<>(original.getCookies().size());
 		copyMultiValueMap(original.getCookies(), this.cookies);