strftime.c: support unknown offset UTC in RFC 3339 [Feature #17544]

In RFC 3339, -00:00 is used for the time in UTC is known, but the
offset to local time is unknown.  Support that representation by
`-` flag for `z`.

strftime.c

@@ -547,7 +547,7 @@ rb_strftime_with_timespec(VALUE ftime, const char *format, size_t format_len,
 			else {
 				off = NUM2LONG(rb_funcall(vtm->utc_offset, rb_intern("round"), 0));
 			}
-			if (off < 0) {
+			if (off < 0 || (off == 0 && (flags & BIT_OF(LEFT)))) {
 				off = -off;
 				sign = -1;
 			}

test/ruby/test_time.rb

@@ -920,6 +920,11 @@ class TestTime < Test::Unit::TestCase
     assert_equal("+09:00", t.strftime("%:z"))
     assert_equal("+09:00:01", t.strftime("%::z"))
     assert_equal("+09:00:01", t.strftime("%:::z"))
+
+    assert_equal("+0000", t2000.strftime("%z"))
+    assert_equal("-0000", t2000.strftime("%-z"))
+    assert_equal("-00:00", t2000.strftime("%-:z"))
+    assert_equal("-00:00:00", t2000.strftime("%-::z"))
   end
 
   def test_strftime_padding