Fix 10.3 invite_email.body assert_match example.

Currently there is no implicit conversion for Mail::Body to string
therefore .to_s must be applied. This is exampled in 10.2.2.
Merge pull request #15989 from nickpellant/master

Fix 10.3 invite_email.body assert_match documentation example.
Don't add a second method for attribute existence

We already had one in the public API that people can use more easily for
the transition