[Fix] Use string instead of GUID for IDs in Payment Service

Author: dougdellolio

CoinbasePro.Specs/Services/Payments/PaymentsServiceSpecs.cs

@@ -36,7 +36,7 @@ class when_requesting_for_all_payment_methods
 
             It should_have_correct_payment_methods = () =>
             {
-                payment_methods.First().Id.ShouldEqual(new Guid("bc6d7162-d984-5ffa-963c-a493b1c1370b"));
+                payment_methods.First().Id.ShouldEqual("bc6d7162-d984-5ffa-963c-a493b1c1370b");
                 payment_methods.First().Name.ShouldEqual("Bank of America - eBan... ********7134");
                 payment_methods.First().Currency.ShouldEqual(Currency.USD);
                 payment_methods.First().AllowBuy.ShouldBeTrue();

CoinbasePro/Services/Payments/Models/PaymentMethod.cs

@@ -8,7 +8,7 @@ namespace CoinbasePro.Services.Payments.Models
 {
     public class PaymentMethod
     {
-        public Guid Id { get; set; }
+        public string Id { get; set; }
 
         [JsonProperty("type")]
         public string PaymentMethodType { get; set; }