add reference document number for invoice corrections

mustang/src/main/java/org/mustangproject/ZUGFeRD/IZUGFeRDExportableTransaction.java

@@ -142,5 +142,12 @@ public interface IZUGFeRDExportableTransaction {
 	 */         
         String getPaymentTermDescription();
         
+        /**
+         * get reference document number
+         * typically used for Invoice Corrections
+         * Will be added as IncludedNote in comfort profile
+         * @return 
+         */
+        String getReferenceNumber();
 	
 }

mustang/src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRDExporter.java

@@ -432,6 +432,13 @@ public class ZUGFeRDExporter {
             regularInfo.getContent().add(regularInfoContent);
             document.getIncludedNote().add(regularInfo);
         }
+        if (trans.getReferenceNumber() != null && trans.getReferenceNumber() != new String()){
+            NoteType referenceInfo = xmlFactory.createNoteType();
+            TextType referenceInfoContent = xmlFactory.createTextType();
+            referenceInfoContent.setValue("Ursprungsbeleg: " + trans.getReferenceNumber());
+            referenceInfo.getContent().add(referenceInfoContent);
+            document.getIncludedNote().add(referenceInfo);            
+        }
 
         return document;
     }

mustang/src/test/java/org/mustangproject/ZUGFeRD/MustangReaderWriterEdgeTest.java

@@ -164,6 +164,11 @@ public class MustangReaderWriterEdgeTest extends TestCase implements IZUGFeRDExp
         return null;
     }
 
+    @Override
+    public String getReferenceNumber() {
+        return null;
+    }
+
   class Contact implements IZUGFeRDExportableContact
   {
 

mustang/src/test/java/org/mustangproject/ZUGFeRD/MustangReaderWriterTest.java

@@ -166,6 +166,11 @@ public class MustangReaderWriterTest extends TestCase implements IZUGFeRDExporta
         //throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
     }
 
+    @Override
+    public String getReferenceNumber() {
+        return null;
+    }
+
   class Contact implements IZUGFeRDExportableContact
   {