Write system chars to correct position in record

The ISO 2709 record builder wrote individually set system chars to the
wrong position in the record label.

Fixes #284

metafacture-biblio/src/main/java/org/metafacture/biblio/iso2709/LabelBuilder.java

@@ -124,7 +124,7 @@ void setSystemChars(final char[] systemChars) {
 
     void setSystemChar(final int index, final char value) {
         assert 0 <= index && index < SYSTEM_CHARS_LENGTH;
-        buffer.setWritePosition(SYSTEM_CHARS_LENGTH + index);
+        buffer.setWritePosition(SYSTEM_CHARS_START + index);
         buffer.writeChar(value);
     }
 

metafacture-biblio/src/test/java/org/metafacture/biblio/iso2709/RecordBuilderTest.java

@@ -183,6 +183,18 @@ public void shouldThrowExceptionIfSystemCharsIsNull() {
         builder.setSystemChars(null);  // Exception expected
     }
 
+    @Test
+    public void shouldWriteSystemCharToRecordLabel() {
+        builder.setSystemChar(0, 'U');
+        builder.setSystemChar(1, 'S');
+        builder.setSystemChar(2, 'C');
+
+        final byte[] record = builder.build();
+
+        assertEquals(0x55, record[17]);
+        assertEquals(0x53, record[18]);
+        assertEquals(0x43, record[19]);
+    }
     @Test(expected = IllegalArgumentException.class)
     public void shouldThrowExceptionIfSystemCharIndexGreaterThan2() {
         builder.setSystemChar(3, '1');