Thumb::width and height methods don't use calculated values

[lukasbestle]

See this post in the forum.

When crop is not set to true, the Thumb class should calculate the missing value right on instantiation so that the values are correct.

[bastianallgeier]

This is solved for all Kirby resizing methods now, but returning a new image object for the thumbnail directly after creation. I plan to overhaul the thumb class as soon as possible. It's quite messy at the moment.