code: function (cb) {
          if (AmazonMws.USER_AGENT_SERIALIZED) {
              return cb(AmazonMws.USER_AGENT_SERIALIZED);
          }
          this.getClientUserAgentSeeded(AmazonMws.USER_AGENT, function (cua) {
              AmazonMws.USER_AGENT_SERIALIZED = cua;
              cb(AmazonMws.USER_AGENT_SERIALIZED);
          });
      }
docs: Gets a JSON version of a User-Agent and uses a cached version for a slight speed advantage.