 Can't set the default value with block syntax in Rails 5
 Can't set the default value with block syntax in Rails 5

Related mattr_accessor change: https://github.com/rails/rails/pull/21158

And now Rails will turn a `nil` "action_dispatch.show_exceptions" to `true` for show_exceptions?, that leads to unexpected behavior for the gem.
https://github.com/rails/rails/blob/afbbcf24d791362a65c7f29b739c73a5b09e3816/actionpack/lib/action_dispatch/http/request.rb#L166-L171
balvig/configurable_exceptions