Cast CGI data to hash, as that's what the code really expects

Fixes an issue where users pass in the `ENV` hash-like object is passed in as
the cgi_data to new Notices.
`ENV.to_hash` [returns a copy](http://www.ruby-doc.org/core-2.1.3/ENV.html#method-c-to_hash), could there by any other cases were not using `dup()` could cause issues here?