filtered params over nested hash

hello there

the Raygun gem has recently introduced the possibility to filter sensitive params, which was highly needed.
however the current implementation doesn't filter over nested hashes - it just filters out top-level keys. it's pretty common in ActiveRecord forms to have nested attributes in forms, like ``user[name]=John&user[password]=secret``. in this case, the ``password`` key wouldn't get filtered.
so I've forked the repo and added this behaviour + a test. nothing fancy :)

please have a look and let me know if this is fits to your gem.

cheers,
Florin.