A CSS identifier can't start with a digit, so when a folder name does,
a '_' character is appended at the beginning of its CSS identifier.
The check for this first character used the `isdigit()` function, which
takes a `char` argument, while `[self objectAtIndex: 0]` returns a
`unichar`, i.e. a 16-bit unsigned integer. This caused some non-digit
characters to pass this check (e.g. Chinese characters), ending up with
an underscore at the beginning of the folder name.
The value of `PidTagResponseRequested` property in the invitation mail
wasn't being set properly, while the `PidTagReplyRequested` property
wasn't being set at all. This caused invitation response mails not to be
sent. Both properties are expected to be `true`.