Fixed about_true_and_false.rb to pass all tests

Learned about/that:
* true is treated as true,
* and false is treated as false,
* nil is also treated as false,
* everything else is treated as true.

[#65851766]
Merge pull request #13 from mkreyman/65851766_about_true_and_false_master

Fixed about_true_and_false.rb to pass all tests

[Delivers #65851766]