change to use post for rapids.json

on utf8 test, I eliminated the chars 0x00 thru 0x1d. When they show up in 'domains' after parse, the json seems to not decode. I used to use those control characters in h2o, so not sure why a problem. Maybe h2o didn't have 'domains' for enums where I was looking post-parse. The only interesting chars in that range are probably CR and LF, and we don't allow embedded CR/LF in enums, so maybe don't care.