Non-Unicode strings in .msg files are encoded using an encoding that is
defined in a separate message property (PR_INTERNET_CPID for the body,
PR_MESSAGE_CODEPAGE for everything else).
The specification says that this property is required, however some real
world .msg files do not have it. This is why the decoding code has a
fallback to "cp1252" (Windows code page 1252, "Western Europe").
fixes#24
