[Image2] If only one dimension is set the missing one should not be set when resizing image

[Piotrek Koszuliński]

TC1:

1. Open image2 sample.
2. Doubleclick image.
3. Change the width to 300.
4. See that height was set too although it was empty at the beginning. I'd expect that it was left empty.

Notice that in step 2 "keep ratio" is on. I think that it's ok, because it tells user that ratio will be kept if he changes size, which is true because the missing dimension is calculated automatically.

TC2:

1. Open image2 sample.
2. Doubleclick image to verify that only width is set. Close the dialog.
3. Resize image using handler.
4. Doubleclick image.
5. See that both dimensions are set now.