Only use valueNormalizer if one is present insideGroup. Fixes #2809

The `valueNormalizer` function is optional in `_HandlebarsBoundView`, see:

  https://github.com/emberjs/ember.js/blob/master/packages/ember-handlebars/lib/views/handlebars_bound_view.js#L246

It should be the same inside of bind's alternative logic for when insideGroup. bind is called without `valueNormalizer` twice, see:

  https://github.com/emberjs/ember.js/blob/master/packages/ember-handlebars/lib/helpers/binding.js#L198
  https://github.com/emberjs/ember.js/blob/master/packages/ember-handlebars/lib/helpers/binding.js#L270

Line 270 is for the `#with` helper, fixing #2809.