Validate availability of json on startup in rlm_rest

If the server was compiled without json support and json was requested in rlm_rest, the server would startup without any problem. However, on the first use of the module, an "assert(false)" was triggered.

This patch validates the bevahiour during the startup phase.
Merge pull request #799 from herwinw/rlm_rest_json_check_on_startup

Validate availability of json on startup in rlm_rest