Verify that MIME::Entity bodies are bytes, and remove _utf8_off call
Use the newly-added RT::Util::assert_bytes function to verify that the body is indeed bytes, and not characters. We also remove the _utf8_off call -- because, contrary to what the comment implies, the presence or absence of the "UTF8" flag does _not_ determine if a string is "encoded as octets and not as characters"; it merely states that the string is capable of holding codepoints > 255. If it happens to not contain any, the _utf8_off does nothing. If it does, it effectively encodes all codepoints > 127 in UTF-8. Given the premise that the string contains bytes in some (probably non-UTF-8) encoding, re-encoding some bytes of it as UTF-8 cannot possibly produce valid output. The flaw in this situation cannot be fixed by a simple _utf8_off, but instead must be fixed by ensuring that the body always contains bytes, not wide characters -- as it now does, thanks to the prior commits. The call to RT::Util::assert_bytes serves as an additional safeguard against backsliding on that assumption.