Inviting an existing user to a site using their username fails to send email

[johnbillion]

Steps to reproduce:

1. Visit the Users -> Add New screen on a blog in a network
2. Add an existing user (who isn't already a user on the current blog) by entering their username in the 'E-mail or Username' field
3. Do not click the 'Skip Confirmation Email' checkbox

After the form is submitted you'll be told that an invite has been sent to the user, but in fact the email won't be sent.

If you repeat the above using the user's email address, it works as expected.

[SergeyBiryukov]

Related: #17219

[johnbillion]

Patch.

[ryan]

In [18965]:

Fix inviting existing users to a site with email confirmation. Props johnbillion. fixes #18295