Add custom attribute support to CreateInvoiceRequest (#8)

* Add custom attribute support to CreateInvoiceRequest

Allow passing one or more custom attributes directly in the invoice
object via setExtra(Map<String, String>). Each entry is serialized
as a top-level key-value pair in the JSON request body.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Add test for custom attribute support in CreateInvoiceRequest

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Update InvoiceGatewayTest.java

* Update InvoiceGatewayTest.java

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Koen Serry <482723+koen-serry@users.noreply.github.com>

Author: 3 people

src/main/java/com/twikey/modal/InvoiceRequests.java

@@ -53,6 +53,7 @@ class CreateInvoiceRequest {
         private String ct;
         private Boolean manual;
         private List<LineItem> lines;
+        private Map<String, String> extra;
 
         public CreateInvoiceRequest(String number, Double amount, String date, String duedate, DocumentRequests.Customer customer) {
             this.number = number;
@@ -138,6 +139,11 @@ public CreateInvoiceRequest setLines(List<LineItem> lines) {
             return this;
         }
 
+        public CreateInvoiceRequest setExtra(Map<String, String> extra) {
+            this.extra = extra;
+            return this;
+        }
+
         /**
          * Converts this request to a Map suitable for API submission.
          */
@@ -180,6 +186,9 @@ public JSONObject toRequest() {
                 for (LineItem line : lines) lineMaps.add(line.toMap());
                 map.put("lines", lineMaps.toString());
             }
+            if (extra != null) {
+                extra.forEach(map::put);
+            }
             return map;
         }
 

src/test/java/com/twikey/InvoiceGatewayTest.java

@@ -76,8 +76,19 @@ public void testCreateInvoice() throws IOException, TwikeyClient.UserException {
         api.invoice().action(actionRequest);
     }
 
-
     @Test
+    public void testCreateInvoiceWithCustomAttribute() throws IOException, TwikeyClient.UserException {
+        Assume.assumeTrue("APIKey is set", apiKey != null);
+        String number = "Inv-CustomAttr-" + System.currentTimeMillis();
+        InvoiceRequests.CreateInvoiceRequest request = new InvoiceRequests.CreateInvoiceRequest(
+                number, 50.0, LocalDate.now().toString(), LocalDate.now().plusMonths(1).toString(), customer)
+                .setExtra(Map.of("myCustomAttribute", "BMW 3-Series"));
+        InvoiceResponse.Invoice response = api.invoice().create(request);
+        assertNotNull("Invoice Id", response.getId());
+        System.out.printf("Created invoice %s with custom attribute%n", response.getId());
+    }  
+    
+    @Test 
     public void testUpdateInvoiceWithCustomAttribute() throws IOException, TwikeyClient.UserException {
         Assume.assumeTrue("APIKey is set", apiKey != null);
         String number = "Inv-UpdateAttr-" + System.currentTimeMillis();