jws: fix base64Decode for strings of length 1 (mod 4).

Change-Id: I5369e4d5454de2cf041363af1328f69d526b17d4
Reviewed-on: https://go-review.googlesource.com/16750
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/jws/jws.go b/jws/jws.go
index e300001..8ca5978 100644
--- a/jws/jws.go
+++ b/jws/jws.go
@@ -161,6 +161,8 @@
 func base64Decode(s string) ([]byte, error) {
 	// add back missing padding
 	switch len(s) % 4 {
+	case 1:
+		s += "==="
 	case 2:
 		s += "=="
 	case 3: