Fix SimpleResizer image ratios when one of the parameters isn't set.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #480
| License       | MIT

When scaling images, the ratio is calculated of both width and height. If one of these isn't set in the context format setting, is is calculated and rounded to an integer value. The ratio is then calculated of this rounded value which gives unexpected results as shown in #480 issue.
Fixes test too, as scaling a 567x200 image to 90px width should return a 90x32 image and not 88x31 (easily checked in any image editor).