Converts data to modified UTF-7
text. Note that data is expected to be
encoded in ISO-8859-1.
This is needed to encode mailbox names that contain certain
characters which are not in range of printable ASCII characters.
The modified UTF-7 encoding is defined in » RFC 2060, section 5.1.3
(original UTF-7 was defined in » RFC1642).
See also: imap_utf7_decode().