Model with no default currency

I noticed an issue with the way money-rails handles attribute currency when no instance nor model default currency is set. In this case it should get and set *_currency* attribute not the global default.

Let me illustrate it with an example:
```ruby
MoneyRails.configure do |config|

  # To set the default currency
  #
  config.default_currency = :eur
end

class PaymentTransaction < ActiveRecord::Base
  attr_accessible :amount_cents, :amount_currency, :tax_cents, :tax_currency, :amount, :tax

  monetize :amount_cents
  monetize :tax_cents

  monetize :total_cents
  def total_cents
    return amount_cents + tax_cents
  end
end

payment_transaction = PaymentTransaction.create(
  amount: Money.new(3210, 'GBP'),
  tax: Money.new(5, 'CHF'))

  payment_transaction.amount_cents.should == 3210
  payment_transaction.amount_currency.should == 'GBP'
  payment_transaction.tax_cents.should == 5
  payment_transaction.tax_currency.should == 'CHF'
```