Rails 3.2 support, remove DoS vector

[oggy]

Hi,

Just added Rails 3.2 support in our branch.

Also removed a potential denial of service vector. Since symbols aren't garbage collected, it's important that we don't simply call #intern / #to_sym on the given value when casting, else malicious user input could in theory fill up the symbol table. Here, I simply maintain a hash of allowed values, so casting simply boils down to a Hash lookup.

Let me know what you think.

Thanks for your work on this!
George