Improve default= handling on MoneyField

[rafalp]

This PR improves our handling for scenario when MoneyField.default is of type Money:

MoneyField.__init__ validates that Money.currency is same as one passed in the currency argument, and raises ValueError otherwise, so its impossible to create model with field as such:

class InvalidModel(models.Model):
    price = MoneyField(
        currency='USD', default=Money('0', 'EUR'), max_digits=9,
        decimal_places=2)

MoneyField.deconstruct tests if default value is instance of Money, and if that is the case, deconstructs kwargs['default'] to str(Money.amount), so migrations generated by Django don't import prices and don't construct explicit Money, which should make them more future-proof.

If somebody out there already has model like in our InvalidModel change, this will be breaking change for his/her project.

[codecov]

Codecov Report

Merging #60 into master will not change coverage.
The diff coverage is n/a.

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  Files           4        4           
  Lines         104      104           
  Branches       10       10           
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  Hits          100      100           
  Misses          2        2           
  Partials        2        2

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[patrys]

Does that mean that we accept integers/strings as default values? Won't that cause even more problems?

[rafalp]

I don't think it would cause errors. str would get passed down to DecimalField.to_python which would happily do Decimal(str) that we would then pass to Money as usual.

Still, our API's shouldn't cause raising eyebrows, and seeing how money becomes string in migration may be causing such eyebrow raise. So I've replaced this with just returning Money.amount, and letting Django take decimal from there.

[patrys]

What would call to_python though? Unless I'm mistaken the constructor does not do that.

[rafalp]

@patrys the path here is roughly Model.__init__() -> MoneyField.get_default() -> MoneyField.to_python() -> DecimalField.to_python.