Fix default value for checkboxes/radios

Reimplement what was done in Phalcon 1.3: https://github.com/phalcon/cphalcon/blob/master/ext/tag.c#L959

Fixes https://github.com/phalcon/cphalcon/issues/3058
Merge pull request #3078 from dugwood/patch-4

[2.0.0] Fix default value for checkboxes/radios