maintain_aspect_ratio parameter not working in image-* plugins

[becky-gilbert]

In the image-* plugins, it seems like resizing an image by specifying either the height or width (via stimulus_height or stimulus_width) with maintain_aspect_ratio: false doesn't work - the image is resized according to the specified dimension, but the other dimension is changed as well and the aspect ratio is maintained. Instead, when maintain_aspect_ratio is false, the unspecified height or width dimension should be the image's original value, and the image should be stretched/compressed accordingly.

I'll fix this in the same pull request that will add the render_on_canvas option to the image plugins (to fix the image flashing issue in Firefox: #891, #969).