Update initialHash after getting defaultDimensions in image model

On initialization, the image model has an initial scalable that has
the given currentDimensions from Parsoid; these are usually correct.
However, in cases where the wiki settings do not fit the user settings
and the images appear smaller or bigger in practice than the values
of the wiki-defaults, the initial hash will store the wrong values.

We will only know what the real values for the comparison will be
after we get them from the API and the calculation; only at that
point we can update the image model initial hash.

This is important so that later the dialog can properly understand
whether to enable the "apply" button if a user changed an image
to custom size and then back to default.

Change-Id: If17b50cc4a39993f98a20a3fec3ddf5d8cb400b3
Merge "Update initialHash after getting defaultDimensions in image model"