Whitelisted disposable domains

[vdolbilov]

Hello,

I added a config option for disposable_domain_with_whitelist which allows for checking only disposable_domain? against the whitelist without checking for MX servers.

Also fixed a spec that was incorrect - is valid if the domain is disposable but in the whitelist was incorrectly passing that user.valid? was falsy due to ValidEmail2.whitelist being cached to an empty set.

I used instance_variable_set in the spec to avoid having to change valid_email2.rb to allow whitelist setting, I can do that instead if you prefer.

[micke]

Thank you!