Action now accounts for updateComment data structure.

[fiatjaf]

This is needed for actions with type updateComment.

[fiatjaf]

I understand all structs in this library were written with the JSON returned by Trello with the default parameters (for example, when fetching /1/cards/<id> without passing any querystring parameter), but since passing arbitrary arguments will now be supported (the plan is to make https://github.com/VojtechVitek/go-trello/pull/14 work for all methods, right?) we will need to make the structs support all kinds of possible data that may come in the responses, correct?

(This PR doesn't address this, it is just a small fix for Action)