extensionDownloader: use POST request for checking updates
Usage of GET requests for checking updates was made deprecated at website some time ago [1], but REST endpoint was CSRF-protected until recently [2].
The body of update request may be big enough and thus does not suitable for GET requests.
[1] Infrastructure/extensions-web@0b38da1b
[2] Infrastructure/extensions-web@e3ab0c07
Edited by Yuri Konotopov