code: function( version ) {
  
      var propName = (this.group ? this.group + '.' : '') + capitalDashCase( this._field )
      var keys = Object.keys( this )
      var params = []
  
      for( var i = 0; i < keys.length; i++ ) {
        if (keys[i] === 'group') continue
        params.push( capitalDashCase( keys[i] ) + '=' + this[ keys[i] ] )
      }
  
      return propName +
        ( params.length ? ';' + params.join( ';' ) : params ) + ':' +
        ( Array.isArray( this._data ) ? this._data.join( ';' ) : this._data )
  
    }
docs: Format the property as vcf with given version
  @param  {String} version
  @return {String}