File downloads no longer work

GatherContent has deprecated the use of "url" within https://docs.gathercontent.com/reference#get-item-files - The correct method of downloading the file is to now call https://docs.gathercontent.com/reference#get-filesfile_iddownload which is authenticated, where as before it wasn't so a standard file_get_contents would suffice.

Downloading files
Although the list files endpoint returns a file URL in the response, this should not be used to download the file content, and should be considered deprecated to be removed in the next API version. The correct way to download the file content is to make an authenticated GET request to /files/{file_id}/download. See docs further down https://docs.gathercontent.com/reference#get-filesfile_iddownload

I contacted GatherContent as they hadn't incremented their API version from 0.5 at all so the depreciation seemed out of the blue. I've provided a patch for 7.x-3.3 which is tested and confirmed works, the implementation isn't the nicest and may cause memory issues if dealing with lots of files but a bit of rewrite would of been required otherwise.

Also attached a patch for the latest dev build on 7.x-3.x-dev but this is untested (due to conflicts on guzzlehttp versions within composer manager), can someone please verify.

I've done a brief check into the latest dev alpha build on D8 and believe the same the same issue exists.