add shortcut '&&=' for preprocessing values only when they exist.

* User `&&=` to preprocess a value, but only if it exists.
    ```Ruby
    #bad
    if params[:tag]
      params[:tag] = params[:tag].downcase
    end

    #ok
    params[:tag] = params[:tag].downcase if params[:tag]

      else
    #better
    params[:tag] &&= params[:tag].downcase
    ```