Fix checkstyle errors

See spring-projectsgh-25771

Author: sdeleuze

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

@@ -79,7 +79,7 @@ public Flux<Object> decode(Publisher<DataBuffer> inputStream, ResolvableType ele
 
 	@Override
 	public Mono<Object> decodeToMono(Publisher<DataBuffer> inputStream, ResolvableType elementType, MimeType mimeType, Map<String, Object> hints) {
-		return stringDecoder
+		return this.stringDecoder
 				.decodeToMono(inputStream, elementType, mimeType, hints)
 				.map(jsonText -> this.json.decodeFromString(serializer(elementType.getType()), jsonText));
 	}

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

@@ -92,7 +92,7 @@ public Flux<DataBuffer> encode(Publisher<?> inputStream, DataBufferFactory buffe
 	@Override
 	public DataBuffer encodeValue(Object value, DataBufferFactory bufferFactory, ResolvableType valueType, MimeType mimeType, Map<String, Object> hints) {
 		String json = this.json.encodeToString(serializer(valueType.getType()), value);
-		return charSequenceEncoder.encodeValue(json, bufferFactory, valueType, mimeType, null);
+		return this.charSequenceEncoder.encodeValue(json, bufferFactory, valueType, mimeType, null);
 	}
 
 	/**