Convert unicode escapse to lowercase (#425)

This aligns with the behaviour exhibited by Ruma & Synapse's
canonicaljson

Author: devonh

json.go

@@ -309,7 +309,7 @@ func compactUnicodeEscape(input, output []byte, index int) ([]byte, int) {
 	}
 	const (
 		ESCAPES = "uuuuuuuubtnufruuuuuuuuuuuuuuuuuu"
-		HEX     = "0123456789ABCDEF"
+		HEX     = "0123456789abcdef"
 	)
 	// If there aren't enough bytes to decode the hex escape then return.
 	if len(input)-index < 4 {

json_test.go

@@ -92,15 +92,15 @@ func TestCompactJSON(t *testing.T) {
 	testCompactJSON(t, input, want)
 
 	input = `["\u0008\u0009\u000A\u000B\u000C\u000D\u000E\u000F"]`
-	want = `["\b\t\n\u000B\f\r\u000E\u000F"]`
+	want = `["\b\t\n\u000b\f\r\u000e\u000f"]`
 	testCompactJSON(t, input, want)
 
 	input = `["\u0010\u0011\u0012\u0013\u0014\u0015\u0016\u0017"]`
 	want = input
 	testCompactJSON(t, input, want)
 
 	input = `["\u0018\u0019\u001A\u001B\u001C\u001D\u001E\u001F"]`
-	want = input
+	want = `["\u0018\u0019\u001a\u001b\u001c\u001d\u001e\u001f"]`
 	testCompactJSON(t, input, want)
 
 	testCompactJSON(t, `["\u0061\u005C\u0042\u0022"]`, `["a\\B\""]`)