quote_token leaves an empty parameter value unquoted, producing invalid "name=" headers #1696

mail 2.9.1, same code on master.

part = Mail::Part.new("Content-Type: image/png; name=\"\"\r\n\r\nx")
part.header[:content_type].decoded  # => "image/png; name="
part.header[:content_type].encoded  # => "Content-Type: image/png;\r\n name=\r\n"

Expected: name="" in the encoded header. Per RFC 2045 section 5.1 a parameter value is a token (one or more characters) or a quoted-string, so an empty value has to be quoted.

Mail::Utilities.quote_token("") returns an empty string because TOKEN_UNSAFE does not match an empty string; ParameterHash#encoded and #decoded use it for every parameter.

Impact: Mail.ru sends pasted images as Content-Type: image/png; name="". Helpdesk software that re-sends such parts (Zammad agent notifications) produces name=, and SMTP servers reject these messages, e.g. 554 Transaction failed: In parameter list <;0x000d0x000a name=>, expected parameter value, got "null".

Possible fix: quote empty values in ParameterHash#encoded. Doing it in quote_token would also change #decoded to name="", and downstream parsers read decoded values: Zammad, for one, would then name such attachments "".