Add missing test.

Author: dampcake

src/test/java/com/dampcake/bencode/BencodeTest.java

@@ -107,6 +107,14 @@ public void testDecodeNullType() {
         instance.decode("12:Hello World!".getBytes(), null);
     }
 
+    @Test
+    public void testDecodeUnknownType() {
+        exception.expect(IllegalArgumentException.class);
+        exception.expectMessage("type cannot be UNKNOWN");
+
+        instance.decode("12:Hello World!".getBytes(), Type.UNKNOWN);
+    }
+
     @Test
     public void testDecodeString() {
         String decoded = instance.decode("12:Hello World!".getBytes(), Type.STRING);