Fix creation of export_fields

issue3219
review874002
(transplanted from 339eb0e9bf5d648ec482a0804198c947fa61749c)

--HG--
extra : transplant_source : 3%9E%B0%E9%BF%5Dd%8E%C4%82%A0%80A%98%C9G%FAat%9C
Use jsonrpc object_hook to load json from URL
(transplanted from ead6182dfa926891e9a429363c23466de561b5f0)

--HG--
extra : transplant_source : %EA%D6%18-%FA%92h%91%E9%A4%296%3C%23Fm%E5a%B5%F0