Suport Array of String for Audience

From the JWT specification, the Audience should accept a array of strings. Whould be nice a methot setAudience that accept a array of Strings and put this attribute as array on JWT As a workaround, I’m setting this as a claim attribute:

String[] aud = { "aud1", "aud2" };
Jwts.builder().claim("aud", aud);
...

Generated payload:

{
  "iss": "issuer",
  "aud": [
    "aud1",
    "aud2"
  ],
  "iat": 1443461450
}

From https://tools.ietf.org/html/rfc7519#section-4.1.3

4.1.3.  "aud" (Audience) Claim

   The "aud" (audience) claim identifies the recipients that the JWT is
   intended for.  Each principal intended to process the JWT MUST
   identify itself with a value in the audience claim.  If the principal
   processing the claim does not identify itself with a value in the
   "aud" claim when this claim is present, then the JWT MUST be
   rejected.  In the general case, the "aud" value is an array of case-
   sensitive strings, each containing a StringOrURI value.  In the
   special case when the JWT has one audience, the "aud" value MAY be a
   single case-sensitive string containing a StringOrURI value.  The
   interpretation of audience values is generally application specific.
   Use of this claim is OPTIONAL.

[jebbench]

It would be good if the requireAudience method could also work with arrays.

[ricveal]

I’ve made a PR #238 that, if it’s accepted, would add support for string arrays in audience attribute without breaks the current functionality. The idea behind the PR is to handle both a simple String and a String array.