Add first currency as default

[antoinekh]

Hello,

As we are not using USD as default currency we want to be able to select the first currency from the ChoiceSet as default as we'd like to avoid changing it every time we create a contract.

With this modification we can used the FIELD_CHOICES and the first element of the list will be proposed as default.

For configuration without FIELD_CHOICES no changed as USD is still the first choice.

    CHOICES = [
        (CURRENCY_USD, 'USD'),
        ('eur', 'EUR'),
        ('chf', 'CHF'),
    ]

[mlebreuil]

Change will be available in version 2.2.8.