Adds specs for Integer#digits
Adds specs for Integer#digits

Specs for Feature [#12447](https://bugs.ruby-lang.org/issues/12447),
Adds Integer#digits.

ruby/spec,ruby/spec,nobu/rubyspec,eregon/rubyspec,nobu/rubyspec,ruby/rubyspec,nobu/rubyspec,eregon/rubyspec,sgarciac/spec,sgarciac/spec,eregon/rubyspec,ruby/spec,ruby/rubyspec,sgarciac/spec