docs: function( input, type, charset ) {
  
      type = ( type || 'Q' ).toUpperCase()
      charset = ( charset || 'utf-8' ).toUpperCase()
  
      if( type === 'B' ) {
        input = this.encodeBase64( input, charset )
      }
  
      input = this.encodeQP( input, true )
      input = [ charset, type, input ].join( '?' )
  
      return '=?' + input + '?='
  
    }