Remove Content-* response headers for error handling

Prior to this commit, when WebFlux handlers added `"Content-*"` response
headers and an error happened while handling the request, all those
headers would not be cleared from the response before error handling.

This commit clears those headers from the response in two places:
* when invoking the handler and adapting the response
* when writing the response body

Not removing those headers might break HTTP clients since they're given
wrong information about how to interpret the HTTP response body: the
error response body might be very different from the original one.

Fixes gh-24238

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -211,7 +211,7 @@ 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);
+			HttpHeaders.clearContentHeaders(exchange.getResponse().getHeaders());
 		}
 
 		InvocableHandlerMethod invocable = this.methodResolver.getExceptionHandlerMethod(exception, handlerMethod);