The idea with HEIM_JSON_F_TRY_DECODE_DATA is that on parsing of JSON
texts, if we find a base64-encoded string, decode it. But a lot of
strings that aren't base64-encoded can be decoded anyways, leaving a
mess.
Insted we should -in a future commit- implement this only for the string
values of "heimdal-type-data-76d7fca2-d0da-4b20-a126-1a10f8a0eae6" names
in singleton objects.