Notification payload hash to hash with access by string keys

Creating Notification from hash with 'aps' key as :symbol instead of 'string' will lose it in payload. That's because 'aps' is read as string key only on initialization. Using HashWithIndifferentAccess would require loading ActiveSupport and would be too big overhead imho...

      notification = Houston::Notification.new(
        :device => push_token,
        :badge => push_badge_counter,
        :sound => 'default',
        :aps => { 'alert' => { 'loc-key' => 'foo', 'loc-args' => ['bar'] }})