Copy header values instead of header lists in DefaultClientRequestBuilder

This commit changes the `headers(HttpHeaders)` method in
DefaultClientRequestBuilder so that it copies the individual header
values instead of using the `List<String>` value directly. The reason
for this change is that the list of values can be immutable, and adding
additional values after that could result in
UnsupportedOperationExceptions.

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientRequestBuilder.java

@@ -73,7 +73,12 @@ class DefaultClientRequestBuilder implements ClientRequest.Builder {
 
 	@Override
 	public ClientRequest.Builder headers(HttpHeaders headers) {
-		this.headers.putAll(headers);
+		for (Map.Entry<String, List<String>> entry : headers.entrySet()) {
+			String headerName = entry.getKey();
+			for (String headerValue : entry.getValue()) {
+				this.headers.add(headerName, headerValue);
+			}
+		}
 		return this;
 	}