fixed #164's bug by focusing on the operator instead of the value

[sio4]

I found that #164's approach is problematic because the fix is focused on the value and it modifies the value itself (from nil to false when if the error is kind of "unknown identifier"). However, since the method is not only for logical operations, the fix made side effects on the evaluation of arithmetic operators and handling nil itself.

This PR has two commits:

• added more test cases
• fixed the method to focus on the operation

fixes #157
fixes #165