Fix bit string problem

When investigating #8206, I found cdd293cb963b895ff580eb20d10f5d56ecb3d447.

I guess that cdd293cb963b895ff580eb20d10f5d56ecb3d447 wasn't properly fix, because we should support bit/hex string.

When using `OID::Bit` for Bit(1560) or Varbit(1562), we support bit/hex string properly.