Unconditionally warn "unknown pack/unpack directive"

[Bug #19150]

pack.c

@@ -167,8 +167,8 @@ unknown_directive(const char *mode, char type, VALUE fmt)
         snprintf(unknown, sizeof(unknown), "\\x%.2x", type & 0xff);
     }
     fmt = rb_str_quote_unprintable(fmt);
-    rb_warning("unknown %s directive '%s' in '%"PRIsVALUE"'",
-               mode, unknown, fmt);
+    rb_warn("unknown %s directive '%s' in '%"PRIsVALUE"'",
+            mode, unknown, fmt);
 }
 
 static float

test/ruby/test_pack.rb

@@ -777,21 +777,13 @@ EXPECTED
   end
 
   def test_pack_garbage
-    assert_warn("") do
-      assert_equal "\000", [0].pack("*U")
-    end
-
-    assert_warning(%r%unknown pack directive '\*' in '\*U'$%) do
+    assert_warn(%r%unknown pack directive '\*' in '\*U'$%) do
       assert_equal "\000", [0].pack("*U")
     end
   end
 
   def test_unpack_garbage
-    assert_warn("") do
-      assert_equal [0], "\000".unpack("*U")
-    end
-
-    assert_warning(%r%unknown unpack directive '\*' in '\*U'$%) do
+    assert_warn(%r%unknown unpack directive '\*' in '\*U'$%) do
       assert_equal [0], "\000".unpack("*U")
     end
   end