Monetizable: Skip subunit write for non-attributes in getter

[edithemmings]

Discovered an issue where making an update to instance currencies causes a MissingAttributeError for converted methods

Example model:

class Transaction < ActiveRecord::Base
  monetize :amount_cents, with_model_currency: :currency
  monetize :tax_cents, with_model_currency: :currency
  monetize :total_cents, with_model_currency: :currency
  monetize :optional_amount_cents, with_model_currency: :currency, allow_nil: true

  def total_cents(foo = 0, bar: 0)
    amount_cents + tax_cents + foo + bar
  end
end

Triggering the error:

transaction = Transaction.create(currency: 'USD')
transaction.update(currency: 'CAD')
ActiveModel::MissingAttributeError: can't write unknown attribute `total_cents`

This occurs because of a condition in read_monetized that tries to update the attribute if the currency changes. This case is needed for maintaining persisted values if the currency changes in a way that the subunit to unit ratio also changes. (This logic is also referred to as being controversial in the TODO comment on the test coverage)

Regardless, the attempt to update the attribute fails for monetized methods because there is no attribute to update. So I added a condition to check for the attribute's presence.