Update helpers.js to allow any value for sets

I'm not sure if this was an oversight after Sets were added, or if this was by design...

Previously any non-numeric value was rejected. This did not allow for adding things like GUIDs or other string values.

I'm not sure of the performance issues here. Perhaps a simple limit on the length of the value would be good, since we don't want tweets injected into the Set. Well...maybe somebody does. :D