code: function(content, undefinedOnly) {
      var k, v;
      for (k in content) {
        v = content[k];
        if (v && typeof v === 'object') {
          content[k] = module.exports.clean(v, undefinedOnly);
          continue;
        }
        if (typeof v === 'undefined') {
          delete content[k];
        }
        if (!undefinedOnly && v === null) {
          delete content[k];
        }
      }
      return content;
    }
docs: Remove undefined and null values