code: function keep(orig, dest, options) {
          var keep, i, t;
          keep = options.keep || [];
          if (keep.length === 0) {
              return dest;
          }
          for (i = 0; i < keep.length; i += 1) {
              t = orig[keep[i]];
              if (t !== undefined) {
                  dest[keep[i]] = t;
              }
          }
          return dest;
      }
docs: Copy the 'options.keep' keys from orig to dest