Ignore ActionController::UnknownFormat by default

After the release of Rails 4 `respond_to` with `respond_with` became throw an exception `ActionController::UnknownFormat` instead of returning a `406 Unacceptable` status directly. However, this is not an exceptional situation requiring notification because the exception is rescued and converted to 406 in the exception handling middleware.

So, i add `ActionController::UnknownFormat` exception to ignored exceptions by default. All test specs has been passed.

What do you think? Thanks!