fix(check commands): Make sure to always pass an array when creating or updating an agent.plugin check.

When creating and updating `agent.plugin` checks, you have to pass the `args` field as an array. #69 fixed things by allowing arrays to be specified, but it doesn't have any way  of knowing that args *always* has to be an array, even if only a single arg is specified. This updates the commands to coerce the field to a list if it isn't one.