Allow overriding of FormHttpMessageWriter#getMediaType

Closes gh-22588

spring-web/src/main/java/org/springframework/http/codec/FormHttpMessageWriter.java

 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -127,8 +127,7 @@ public class FormHttpMessageWriter extends LoggingCodecSupport
 		mediaType = getMediaType(mediaType);
 		message.getHeaders().setContentType(mediaType);
 
-		Charset charset = mediaType.getCharset();
-		Assert.notNull(charset, "No charset"); // should never occur
+		Charset charset = mediaType.getCharset() != null ? mediaType.getCharset() : getDefaultCharset();
 
 		return Mono.from(inputStream).flatMap(form -> {
 			logFormData(form, hints);
@@ -140,7 +139,7 @@ public class FormHttpMessageWriter extends LoggingCodecSupport
 		});
 	}
 
-	private MediaType getMediaType(@Nullable MediaType mediaType) {
+	protected MediaType getMediaType(@Nullable MediaType mediaType) {
 		if (mediaType == null) {
 			return DEFAULT_FORM_DATA_MEDIA_TYPE;
 		}