Image resize did not work on WPML sites

Hi,

tl;dr: Using the resize filter on a site with WPML translations resulted in broken resized image links, I fixed it (and added non-regression tests).

More details:
- WPML works by changing home_url() from 'example.com' to 'example.com?lang=en'
- timber resize used to build resized url by doing home_url().'...path...'
- mixing the 2 resulted in broken url of 'example.com **?lang=en** /wp-content/.../img-300x500-c-default.jpg'
- I updated resize to use content_url(), which WPML does not touch
- I very slightly refactored timber-image-helper in the process.

Thanks