Upgrade Webmock to 3.5.1
Webmock 3.5.0 brings Ruby 2.6 support.
This MR also fixes BitbucketServer::Client
spec failing with Webmock 3.5.1.
Webmock 3.1.0 changed the behavior to return nil
for the body if an
HTTP 204 No Content response were received
(https://github.com/bblimke/webmock/commit/b837e642782501a6904d0cb1aad391dd6f49ada3).
Update the Bitbucket Server connection to ignore these No Content response codes.
Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/57323