quote empty string

there be some pseudo codes.
str = {"empty":"","docId":111,"serviceName":"cafe"}
map = parser.parse(str);
print serializer.serialize(map)
{"empty":,"docId":111,"serviceName":"cafe"}

even if set includeEmpty().includeNulls().includeDefaultValues() methods.
this bug(?) is shown on 0.10 and 0.11-SNAPSHOT(from github).