AvroAsTextInput format creates invalid JSON for NaN values

The AvroAsTextInput format generates a record containing

[omitted] , "someName": NaN}

A json parser chokes on that and it is not legal JSON AFAIK. In related issues AVRO-972 and AVRO-1290 it is resolved, for lack of a better option, to map the value NaN to the string "NaN", which needs to be quoted in JSON. Our parser can handle that with no modifications, Java will parse such a string to the correct value using standard libraries.