See Stripe API reference for more information about Coupon
create(map) :: {:ok, map} | {:error, binary}
delete(binary) :: {:ok, map} | {:error, binary}
index() :: {:ok, map} | {:error, binary}
index(map) :: {:ok, map} | {:error, binary}
show(binary) :: {:ok, map} | {:error, binary}
update(map) :: {:ok, map} | {:error, binary}
update(binary, map) :: {:ok, map} | {:error, binary}