Don't consider substrings when checking distinct mailbox names
RFC822.MailboxAddress.has_distinct_name() really needs to not do
sub-string checks since it will cause addresses like
"foo-bar@baz" <bar@baz>
to not have a distinct name. To keep the fix
for #491 (closed) in effect, also adds special case checking for sinlge quotes.
Add some more tests to cover these cases.
Partially reverts commit 6e137eb6
See discussion in !252 (merged)