Bug: `isEmail` docs/source suggest `allow_underscores` is defaulted to `false` however `foo_bar@gmail.com` is valid by default

[titanism]

As described in title.

[jamro]

I think this is expected behaviour. According to RFC 5322, an underscore can appear in the local-part of the email address and is a valid character (when used before @). The underscore is not allowed in the domain name; however, there are some cases where it could appear in a subdomain. This is what allow_underscores flag is for.

If you want to exclude underscore from local-part of the email, use blacklisted_chars option:

validator.isEmail('foo_bar@gmail.com', { blacklisted_chars: '_' }) // false

[titanism]

@jamro see #2508