Remove response content-type before error handling

Prior to this commit, the negotiated content-type during the request
mapping phase would be kept as the response content-type header; this
information is used when rendering the error response and prevents a new
round of content negotiation to choose the media type that fits best.

This commit removes the response content type information at the
beginning of the error handling phase.

Fixes gh-22452

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerAdapter.java

@@ -29,6 +29,7 @@ import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.ReactiveAdapterRegistry;
+import org.springframework.http.HttpHeaders;
 import org.springframework.http.codec.HttpMessageReader;
 import org.springframework.http.codec.ServerCodecConfigurer;
 import org.springframework.lang.Nullable;
@@ -209,6 +210,9 @@ public class RequestMappingHandlerAdapter implements HandlerAdapter, Application
 
 		// Success and error responses may use different content types
 		exchange.getAttributes().remove(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
+		if (!exchange.getResponse().isCommitted()) {
+			exchange.getResponse().getHeaders().remove(HttpHeaders.CONTENT_TYPE);
+		}
 
 		InvocableHandlerMethod invocable = this.methodResolver.getExceptionHandlerMethod(exception, handlerMethod);
 		if (invocable != null) {