Problem with TOTP

[Skyost]

Hey,

There's a little problem with the TOTP class. If it generates a number starting with a 0, it is not taken into account by Dart. For instance : the class generates 012345 and Dart returns 12345.

The only possible fix would be to return a String instead of an int. We could avoid a breaking change by adding a stringValue() method that returns a String padded with as many 0 as needed to match the TOTP length.