utils: ensure TextDecoder is only removing utf-8 BOM on utf-8 encoding

Author: phated

lib/internal/encoding.js

@@ -512,6 +512,7 @@ function makeTextDecoderJS() {
         this[kHandle].write(input);
 
       if (result.length > 0 &&
+          this[kEncoding] === 'utf-8' &&
           !this[kBOMSeen] &&
           !(this[kFlags] & CONVERTER_FLAGS_IGNORE_BOM)) {
         // If the very first result in the stream is a BOM, and we are not