Collecting platform fees with Connect destination charges

[damirstuhec]

As already discussed with @anthonycastelli and @Andrewangeta, the platform fee collection when creating destination charges is currently broken.

The documentation says that this should be achieved by simply specifying the total amount, together with destination[account] and destination[amount] where latter should be calculated by subtracting the platform’s fee from the total charge amount. No explicit "application fee" parameter is needed.

Please find my PR that fixes this issue referenced below. ✌️