code: function( prop ) {
  
      var key = prop._field
  
      if( Array.isArray( this.data[ key ] ) ) {
        this.data[ key ].push( prop )
      } else if( this.data[ key ] != null ) {
        this.data[ key ] = [ this.data[ key ], prop ]
      } else {
        this.data[ key ] = prop
      }
  
      return this
  
    }
docs: Add a vCard property from an already
  constructed vCard.Property
  @param {vCard.Property} prop