function_generators: evaluate_strictness: return 0 if function succeeded

[nicola-lunghi]

If the [[ a =~ b]] comparison return false, the function returns false as well. This makes the script exit if I set the errexit options if the input are valid

This fixes issue #163

[matejak]

Could you please extend your PR so it adds the corresponding check to the test suite?
AFAIK, your use case corresponds to e.g. setting set -e as in https://github.com/matejak/argbash/blob/master/tests/regressiontests/basic.m4. Would it be possible to extend that script, so it expands that offending code?

Thank you for your PR, and don't be shy to add your name to AUTHORS as well.

[nicola-lunghi]

fixed in #173