Allow iiurlheight to be used without iiurlwidth

Allow iiurlheight to be used without iiurlwidth

Wanting to get a thumbnail with only the height constrained is a
reasonable use case, the only reason we don't support it in the
API is because the backend doesn't support it directly. This is
easy enough to emulate by setting the width to the full width of
the image if only the height is specified. This makes the transform
function use a bounding box of size w x h, and if w is the width of
the original image that's equivalent to just bounding the height.

Change-Id: I28c7c22ee91669469cbd9e7d25f09100933582de