This change broke the nightly builds for Debian Oldstable. The most recent version of libsoup available in Debian Oldstable is 2.38 (https://packages.debian.org/wheezy/libsoup2.4-dev).
I think we can either keep the code more backwards compatible and revert the change or add some guards for older libsoup versions or we just keep the requirement on newer libsoup versions for this plugin and disable it in nightly builds for Debian Oldstable.