Use BODY instead of RFC822

RFC3501 seems to prefer BODY attributes as the more modern equivalent
to RFC822. For example, this also allows us to use BODY.PEEK. Since the
fetch methods in melib are tightly coupled with the parser, we have to
add this here if we want to use BODY attributes during fetch.

melib/src/backends/imap/protocol_parser.rs

@@ -624,8 +624,8 @@ pub fn fetch_response(input: &[u8]) -> ImapParseResult<FetchResponse<'_>> {
                     String::from_utf8_lossy(input)
                 ))));
             }
-        } else if input[i..].starts_with(b"RFC822 {") {
-            i += b"RFC822 ".len();
+        } else if input[i..].starts_with(b"BODY[] {") {
+            i += b"BODY[] ".len();
             if let Ok((rest, body)) =
                 length_data::<_, _, (&[u8], nom::error::ErrorKind), _>(delimited(
                     tag("{"),