Allow "Use Original" for Resolution when exporting images from LightRoom

In Lightroom, when doing an export, you currently have to select a single Resolution (i.e., PPI) for all images. However, you may be processing a number of images with different PPI, and would like to retain the original images values.

I propose that in the Export dialog of Lightroom, that the the 'Resolution' field under Image Sizing' have a 3rd option in the combobox: "Retain original". If selected, then the resolution number box becomes disabled, and all exported images will have the same resolution (PPI) as the original file.

For example:
I have 1 file that is 300x300 pixels, at 300PPI. I have another file that is 600x600 pixels, at 600PPI. If I just open and print them from Photoshop, they are both printed at 1"x1". If I then Develop them in Lightroom, Export them, I am now forced to select a PPI value, such as 300PPI. If I do, and repeat the printing process with the exported images, my 600x600 image will now print at 2"x2" (I don't want to manually correct the file before printing).