OSRA-207 Add 'Payment Plan' to Sponsor

OSRA-207
@PurityControl :+1:
`.to be_present` (no `?`) and `.not_to be_empty` check that the constant is defined *and* that the array is not empty. `.to be_present` is the only one that guards against `['']`. This might be overkill as things would have to have gone way off the track for that array to contain an empty string, but technically it is indeed the best solution. `.to be_truthy`, `.to be`, `.not_to be_nil` are all green when the array is empty. Thanks for the education!