Only pass crop parameter when ratio is specified

[hirasso]

Hi @tobimori ,

thanks so much for this beautiful plugin!

Today I suddenly ran into memory issues. While source-diving and playing around, I noticed that in my case, the $file->thumb() doesn't actually do any downsizing when cropping is set to true, leading to Memory allocation errors.

https://github.com/tobimori/kirby-thumbhash/blob/cfc48966eb901d3b6a80142385ca70415d697073/classes/ThumbHash.php#L45

Setting that to false magically fixed it. While this of course doesn't really make sense and seems to be pointing towards an underlying issue in my Kirby install, I'm still wondering what the reasoning is behind setting cropping to true?