total_amount is not calculated when a line item is added

[etcook]

Because the calculate_total_amount callback is only run on before_validation, the amount never gets updated unless you update the invoice. Even touching it or using something like AASM to change state won't trigger the callback to be run.

This should probably be changed to a more liberal callback or at least supported with a callback on line_item that gets triggered on save.

[etcook]

@iffyuva This is one I'd like to address, however, any implementation will be opinionated so I'd like to get feedback first. Are we trying to avoid invoice total calculation with each line_item save for performance reasons?

[iffyuva]

@etcook I don't remember the reason for not incorporating this. Only think I can think of is: Supporting older rails versions.

I'm happy to add this behavior. Please open a PR whenever you are free. Thanks.