Respect `$akHandle` in RatingHelper

Respect `$akHandle` parameter in
`RatingHelper::getAverageChildRating()`

Allows for using rating helper with custom attributes
Add Default `$akHandle` 'rating' to `RatingHelper`

- Add default `$akHandle = rating` to
  `RatingHelper::getAverageChildRating()
- Add default `$akHandle = rating` to
  `RatingHelper::outputAverageChildRating()
Merge pull request #1144 from mkly/check_for_rating_attribute_handle

Respect `$akHandle` in RatingHelper