Raise argument errors correctly in lua redis.call calls.

This fixes issue #651 "redis.call doesn't throw lua error when passed nil arguments". I had some trouble debugging a script which came down to me inadvertently passing a boolean to a redis.call. I was confused as to why I was getting a table back from an HGET call. I understand if this is intentional, but it would be nice for the documentation to be updated in that case to be more clear.