Instead of doing a method lookup and doing send, why don't we have something like this?

```ruby
  interpolated_data = perform_interpolation(data, interpolation_type)
  [ ... ]
end

def perform_interpolation(data, scope, interpolation_type)
  case interpolation_type
  when :hiera
    hiera_interpolate(data, scope)
  when :scope, NilClass
    scope_interpolate(data, scope)
  else
    raise Hiera::UnknownInterpolation, "Could not interpolate #{data.inspect}: #{interpolation_type} not a recognized interpolation type"
  end
end
```