MapReduce functions: four-byte characters are replaced by U+FFFD

I have some documents, containing properties, whose value contains four-byte UTF8 characters. Those are mostly emojis.

Lately, I found out, that those characters are not correctly handled if I add them to the result of a map-reduce method. If I use it inside a map-reduce function, all those characters are returned as U+FFFD, called REPLACEMENT CHARACTER.

Steps to reproduce:

1. Add a document containing an emoji. Sample: { "id": 1, "name": "test 😊" }
2. Add a map-reduce method, that returns that one. Sample: function (doc, meta) { emit(doc.id, doc.name); }
3. Call that map-reduce method and take a look at the result

A workaround, I use currently, is to fetch the document by the key-value-interface. That one returns the emojis correctly.