[TRANSFERS] Calling setHolder before setTransfers causes a warning.

[joaolucasl]

In setHolder there's a reference to transferInstrument, which is instantiated in setTransfers.

https://github.com/moip/moip-sdk-php/blob/8b13518d19cc4f8ab8285357f05c4d6d0d57da07/src/Resource/Transfers.php#L93-L100

If this method is called before setTransfers, it will trigger a warning.

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How to fix?

Both setHolder and setTransfers need to check for the existence of $this->data->transferInstrument - if it doesn't exist, it is instantiated. (Otherwise, it would override the existing data)