This issue was solved with changeset r37041.
Masaki, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
io.c: IO#reopen int mode
io.c (rb_io_reopen): accept File::Constants as well as mode string.
based on the patch by Glass_saga (Masaki Matsushita) in [ruby-core:47694]. [Feature #7067]