Escape a backslash in a quoted display name

[pcbeingused333]

Address.with_name quotes a display name when it contains a special character, but quote_escaped only escaped " inside the quoted string — never \. RFC 5322 requires both to be escaped as a quoted-pair.

Courrier::Email::Address.with_name("d@example.com", "Smith, John\\")
# => "\"Smith, John\\\" <d@example.com>"
#        the trailing \ escapes the closing " — the quoted string runs on
#        into the address and the header is corrupt

Courrier::Email::Address.with_name("d@example.com", "C:\\Users, Inc")
# => "\"C:\\Users, Inc\" <d@example.com>"
#        \U is a quoted-pair; a parser reads the name as "C:Users, Inc"

Fix

• quote_escaped escapes \ as well as ".
• \ joins the set in special_characters_in?, so a bare backslash in an otherwise plain name (Doe\) is quoted instead of going out as an unquoted atom, which is also illegal.

Tests

Two cases added to address_test.rb: a backslash mid-name and a trailing backslash. bundle exec rake test on Ruby 3.4: 161 runs, 304 assertions, 0 failures. rake standard clean.

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