Fix using sales invoice custom fields

[benjaminwols]

Moneybird has changed the way custom_fields are used
This PR adds a custom_field resource the same way as event/detail is used

Changelog 09-08-2019

The custom_fields_attributes and details_attributes fields are now all consistent with each other. This simplifies the mapping of there between different endpoints. It has changed from custom_fields_attributes[*][id] to custom_fields_attributes[id].

[maartenvanvliet]

👍
Just curious, are you using the gem anywhere (in production)?

[benjaminwols]

Not yet, but we are integrating at the moment and are planning to use it in production.