Reload doesn't always work correctly with money_column

[sunblaze]

Just noticed this while reviewing #90

We set an instance variable when the currency is fixed or is readonly, but we don't clear the instance variable on a reload of the model.

Here's an example test of the issue:

original = MoneyRecord.create!(price_usd: Money.new(1, 'USD'))
copy = MoneyRecord.last
copy.update!(price_usd: Money.new(2, 'USD'))
original.reload
assert_equal Money.new(2, 'USD'), original.price_usd

@elfassy