Allow Ds_Merchant_Identifier parameter in PurchaseRequest

[danipct]

Hello,

It is possible to add Ds_Merchant_Identifier parameter in PurchaseRequest class? I need it to store card references saved by user.

Thank you,

[leith]

Looking at the parameter docs, this is an optional field with this description:

This parameter indicates the card reference to use, or a request to generate a reference.
To generate a new reference, send "REQUIRED". In the case of SOAP or REST web services, the DS_MERCHANT_PAN parameter is mandatory.

So for a PurchaseRequest (the redirect gateway), this is used as a card token field, for adding saved card support.

In Omnipay this would map to getToken() and/or getCardReference() if it's a supplied card reference. If you're requesting a new reference there isn't a specific standard method for this, but a few gateways use a createToken (bool) parameter to indicate if a new card should be made (as per my PR to common), so I'd probably take the same approach here and map them both back into Ds_Merchant_Identifier.

@danipct happy to review a PR if you have the time, otherwise we'll hopefully get to it soon.