Updated the Regex method. Issue #421 Fixed

[ArielBlanco1990]

Updated the Regex method that validates the email address when performing the Checkout, allowing the address to contain the underscore character in the address. Only allowed in the part of the email before the @domain... The Regex method that DNN uses when validating the email record allows this character. #421

[ArielBlanco1990]

Regex method for mail validation used by DNN when doing a user registration

[ArielBlanco1990]

I have recreated the moment of creating a payment before and after this update using the email referenced in the Issue #421. The results are shown below.

Before:

Later: