Lookup for email in username field to match docs if email is undefined

[flovilmart]

Fixes #2502

When the email is not set on the user, the docs expressively says that the server should lookup in the username field.

This PR propose that change.

Note that the adapters should adapt to that change if implementing sendPasswordResetEmail, and lookup in the username field is email is not set.

If the adapter like parse-server-simple-mailgun-adapter only implements sendMail no change is required.

[ghost]

@flovilmart updated the pull request - view changes