mailinfo: don't insert header prefix for handle_content_type()

handle_content_type() only cares about the value after "Content-Type: ";
there is no need to insert that string for it.

Suggested-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mailinfo.c

@@ -570,7 +570,6 @@ static int check_header(struct mailinfo *mi,
 		len = strlen("Content-Type: ");
 		strbuf_add(&sb, line->buf + len, line->len - len);
 		decode_header(mi, &sb);
-		strbuf_insertstr(&sb, 0, "Content-Type: ");
 		handle_content_type(mi, &sb);
 		ret = 1;
 		goto check_header_out;