Enhancement: with ".disableAllClientValidation(true)", also ignore errors from the completeness check

[bbottema]

Originally reported in #448. According to RFC 5322, the From header and recipients list is mandatory for sending emails. However, there are use cases where this rule should not be enforced. For example when only logging an email, or when handling it using a CustomMailer.

There's a .disableAllClientValidation(true) method already, but this precludes the completeness check. This check should now also be ignored (or at least made only to log issues as warnings).

[bbottema]

v8.0.0 released. @morki, it would be great if you could verify!

[morki]

@bbottema thank you very much, it works perfectly even without the workaround :)