allow_blank does not work

[cs0511]

Current code only checks when allow_blank is true, it doesn't check if allow_blank is false.

If an attribute allows String, but doesn't want to allow empty string, allow_blank doesn't work at all.

[stevehanson]

This would be useful on my project as well. We have been having to use regex types (eg. /.*\S.*/) instead of String in order to validate that the string is not blank.

[mathieujobin]

I think I have been bitten by this once as well...
but 3 tests are failing for rails 4.2
unless we can fix them, it means we would remove support and cut a major version