html/template: ensure that MIME type handling is case insensitive

Handle MIME types found in the type attribute of the script element
in a case insensitive way, as per Section 5.1 of RFC 2045.

Fixes #19968

Change-Id: Ie1416178c937dcf2c96bcec4191cebe7c3477af8
Reviewed-on: https://go-review.googlesource.com/40702Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/html/template/escape_test.go

@@ -1399,6 +1399,11 @@ func TestEscapeText(t *testing.T) {
 			`<script type="text/template">`,
 			context{state: stateText},
 		},
+		// covering issue 19968
+		{
+			`<script type="TEXT/JAVASCRIPT">`,
+			context{state: stateJS, element: elementScript},
+		},
 		{
 			`<script type="notjs">`,
 			context{state: stateText},

src/html/template/js.go

@@ -372,7 +372,7 @@ func isJSType(mimeType string) bool {
 	//   https://tools.ietf.org/html/rfc7231#section-3.1.1
 	//   https://tools.ietf.org/html/rfc4329#section-3
 	//   https://www.ietf.org/rfc/rfc4627.txt
-
+	mimeType = strings.ToLower(mimeType)
 	// discard parameters
 	if i := strings.Index(mimeType, ";"); i >= 0 {
 		mimeType = mimeType[:i]