Ensure no leading zero byte on RSA mod

[Connect2id OSS]

http://www.ietf.org/mail-archive/web/jose/current/msg04297.html

6.3.1.1. "n" (Modulus) Parameter


   The "n" (modulus) member contains the modulus value for the RSA
   public key.  It is represented as the base64url encoding of the
   value's unsigned big endian representation as an octet sequence.  The
   octet sequence MUST utilize the minimum number of octets to represent
   the value.

[Connect2id OSS]

Added test in commit 2e305a3, no fix required.