mime/multipart: check for quoted-printable case insensitively

Fixes #28674

Change-Id: Id88e0a4b86b50eb45f0d968d7e4bbe66b7f37f82
Reviewed-on: https://go-review.googlesource.com/c/148579
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/mime/multipart/multipart.go

@@ -21,6 +21,7 @@ import (
 	"mime"
 	"mime/quotedprintable"
 	"net/textproto"
+	"strings"
 )
 
 var emptyParams = make(map[string]string)
@@ -135,7 +136,7 @@ func newPart(mr *Reader) (*Part, error) {
 	}
 	bp.r = partReader{bp}
 	const cte = "Content-Transfer-Encoding"
-	if bp.Header.Get(cte) == "quoted-printable" {
+	if strings.EqualFold(bp.Header.Get(cte), "quoted-printable") {
 		bp.Header.Del(cte)
 		bp.r = quotedprintable.NewReader(bp.r)
 	}

src/mime/multipart/multipart_test.go

@@ -419,8 +419,16 @@ func TestLineContinuation(t *testing.T) {
 }
 
 func TestQuotedPrintableEncoding(t *testing.T) {
+	for _, cte := range []string{"quoted-printable", "Quoted-PRINTABLE"} {
+		t.Run(cte, func(t *testing.T) {
+			testQuotedPrintableEncoding(t, cte)
+		})
+	}
+}
+
+func testQuotedPrintableEncoding(t *testing.T, cte string) {
 	// From https://golang.org/issue/4411
-	body := "--0016e68ee29c5d515f04cedf6733\r\nContent-Type: text/plain; charset=ISO-8859-1\r\nContent-Disposition: form-data; name=text\r\nContent-Transfer-Encoding: quoted-printable\r\n\r\nwords words words words words words words words words words words words wor=\r\nds words words words words words words words words words words words words =\r\nwords words words words words words words words words words words words wor=\r\nds words words words words words words words words words words words words =\r\nwords words words words words words words words words\r\n--0016e68ee29c5d515f04cedf6733\r\nContent-Type: text/plain; charset=ISO-8859-1\r\nContent-Disposition: form-data; name=submit\r\n\r\nSubmit\r\n--0016e68ee29c5d515f04cedf6733--"
+	body := "--0016e68ee29c5d515f04cedf6733\r\nContent-Type: text/plain; charset=ISO-8859-1\r\nContent-Disposition: form-data; name=text\r\nContent-Transfer-Encoding: " + cte + "\r\n\r\nwords words words words words words words words words words words words wor=\r\nds words words words words words words words words words words words words =\r\nwords words words words words words words words words words words words wor=\r\nds words words words words words words words words words words words words =\r\nwords words words words words words words words words\r\n--0016e68ee29c5d515f04cedf6733\r\nContent-Type: text/plain; charset=ISO-8859-1\r\nContent-Disposition: form-data; name=submit\r\n\r\nSubmit\r\n--0016e68ee29c5d515f04cedf6733--"
 	r := NewReader(strings.NewReader(body), "0016e68ee29c5d515f04cedf6733")
 	part, err := r.NextPart()
 	if err != nil {