message: docs(headers): clarify Basic auth
  See
diff: @@ -140,7 +140,8 @@ pub struct Basic {
  /// The username as a possibly empty string
  pub username: String,
  /// The password. `None` if the `:` delimiter character was not
  - /// part of the parsed input.
  + /// part of the parsed input. Note: A compliant client MUST
  + /// always send a password (which may be the empty string).
  pub password: Option<String>
  }
  