Fix: boolean rule to check if value is a boolean

[jonwaldstein]

Resolves: #16

This prevents the boolean rule from returning false positives by adding an is_bool check and FILTER_NULL_ON_FAILURE flag to filter_var.

After using filter_var to determine the boolean value we use is_bool to check if the value is a boolean. With the addition of the filter flag FILTER_NULL_ON_FAILURE,filter_var will now return null on failure - causing is_bool to return false.

[jonwaldstein]

@JasonTheAdams this is ready for review.

[JasonTheAdams]

Good catch, @jonwaldstein! And thanks for updating the tests!