Refactor iframe validate to match servlet API

The servlet API supplies POST parameters as a `Map<String, String[]>`, but ToopherIframe.validate took a `Map<String, String>`.  This means that requesters had to preprocess the parameters before calling `validate`, which is a pain.

I'd like to be able to just give a function overload so `validate` could accept either data structure, but I can't, because Java.

*edit* - fixed markup so it wouldn't say "Change Map to Map"