Added parameters option to api check request handler

I had a requirement to be able to provide some parameters to a check via the check request API. Typically useful for unpublished, ad-hoc checks where you want to issue a command to a set of clients but set some parameters to that command at run-time.

Not a ruby expert, but this seems like the simplest approach that's tolerant of cases where the command has a placeholder but no params are supplied, or vice versa.