send-email: avoid empty transfer encoding header

Fix a small bug introduced by "7a36987ff (send-email: add an auto option
for transfer encoding, 2018-07-14)".

I saw the following message when setting --transfer-encoding for a file
with the same encoding:

    $ git send-email --transfer-encoding=8bit example.patch
    Use of uninitialized value $xfer_encoding in concatenation (.) or string
    at /usr/lib/git-core/git-send-email line 1744.

The new tests are by brian m. carlson.

Signed-off-by: Aaron Lindsay <aaron@aclindsay.com>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1835,7 +1835,7 @@ sub apply_transfer_encoding {
 	my $from = shift;
 	my $to = shift;
 
-	return $message if ($from eq $to and $from ne '7bit');
+	return ($message, $to) if ($from eq $to and $from ne '7bit');
 
 	require MIME::QuotedPrint;
 	require MIME::Base64;

t/t9001-send-email.sh

@@ -493,6 +493,21 @@ do
 			--validate \
 			$patches longline.patch
 	'
+
+done
+
+for enc in 7bit 8bit quoted-printable base64
+do
+	test_expect_success $PREREQ "--transfer-encoding=$enc produces correct header" '
+		clean_fake_sendmail &&
+		git send-email \
+			--from="Example <nobody@example.com>" \
+			--to=nobody@example.com \
+			--smtp-server="$(pwd)/fake.sendmail" \
+			--transfer-encoding=$enc \
+			$patches &&
+		grep "Content-Transfer-Encoding: $enc" msgtxt1
+	'
 done
 
 test_expect_success $PREREQ 'Invalid In-Reply-To' '