class ActionController::Parameters

object per fetch.
loop that converts values. Also, we are not going to build a new array
Testing membership still loops, but it's going to be faster than our own

method to instantiate it only if needed.
looping in the common use case permit + mass-assignment. Defined in a
Attribute that keeps track of converted arrays, if any, to avoid double

def converted_arrays
  @converted_arrays ||= Set.new
end