send-email: add transfer encoding header with content-type

We add the content-type header only when we have non-7bit
characters from the 'From' header, so we really need to
specify the encoding (in other cases, where the commit text
needed a content-type, git-format-patch will already have
added the encoding header).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -711,7 +711,8 @@ sub send_message
 			else {
 				push @xh,
 				  'MIME-Version: 1.0',
-				  "Content-Type: text/plain; charset=$author_encoding";
+				  "Content-Type: text/plain; charset=$author_encoding",
+				  'Content-Transfer-Encoding: 8bit';
 			}
 		}
 	}