Added Multipass properties

[hardikcc]

Hi,

I have additional properties to multipass request. New properties are:

tag_string
identifier,
remote_ip,
return_to

I have also fixed the test to utilize the MultipassSecret instead of ShopifyToken.

Please merge.

[nozzlegear]

Hey @hardikcc thanks for the pull request! Just to clarify, are these new properties required for using MultipassService.GetMultipassUrl, i.e. the call to Shopify would fail without them? If they're not required I'd like to add an overload instead that will accept the previous Customer class and a new one that will accept the MultipassRequest class.

[hardikcc]

Hello @nozzlegear, these are optional parameters. GetMultipassUrl will definitely work without them. Let me make these changes.

[nozzlegear]

@hardikcc Awesome, thanks! Let me know when you've got them ready and I'll get it merged in.