Relationship fields are not saved when submitting a relationship form with WPML active

Symptoms

When submitting a relationship form for a many-to-many relationship that includes custom fields for the intermediary post, those custom fields will not get properly saved if WPML is active on the site.

Workaround

This issue will get fixed in the next release for Toolset Forms. Until this happens, this patch can be used.

To apply it, just unzip the package and upload the file it contains, replacing the one with the same name, located in /application/model/association_forms/.