html/template: lock in application/json as valid JS test

CL https://go-review.googlesource.com/33899 added
application/json as a mimeType for valid JS. Let's
lock that fix in with a test.

Updates #18159

Change-Id: Ic4dfd8929aebfc5410f796688f081ca06630f672
Reviewed-on: https://go-review.googlesource.com/33901
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Nodir Turakulov <nodir@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/html/template/js_test.go

@@ -341,6 +341,7 @@ func TestIsJsMimeType(t *testing.T) {
 		{"application/javascript;version=1.8;foo=bar", true},
 		{"application/javascript/version=1.8", false},
 		{"text/javascript", true},
+		{"application/json", true},
 	}
 
 	for _, test := range tests {