[#32] Documents: Add PaymentVariantSummary

Elwig/Documents/Document.cs

@@ -88,6 +88,8 @@ namespace Elwig.Documents {
                 name = "MemberList";
             } else if (this is WineQualityStatistics) {
                 name = "WineQualityStatistics";
+            } else if (this is PaymentVariantSummary) {
+                name = "PaymentVariantSummary";
             } else {
                 throw new InvalidOperationException("Invalid document object");
             }