Fix GRPCUseTLS flag HTTP API mapping

[amenzhinsky]

Currently HTTP APi doesn't recognize snake-cased grpc_use_tls option whereas loading from a config file works as expected, this fixes the following:

curl -X PUT -d '{
  "name": "test",
  "port": 6060,
  "check": {
    "grpc": "127.0.0.1:8080/test",
    "grpc_use_tls": true,
    "interval": "5s"
  }
}' http://127.0.0.1:8500/v1/agent/service/register

Request decode failed: json: unknown field "grpc_use_tls"

I know there was a lot of copy-pasting just to make it work 😢 probably it can be improved later for all fields.

Can be worked around now: "GRPCUseTLS": true

[dnephin]

Thank you for this fix!

Looks good! I think the only thing that is missing is a changelog entry. I've pushed a commit with the changelog entry.