Remove circular dependency between HTTPHelper and GitlabNet
HttpHelper depended on exceptions defined in GitLabNet, but GitLabNet
included HttpHelper. Specs in Ruby 2.5 were breaking as a result of
Action::Custom not able to reference constants in GitLabNet:
https://gitlab.com/gitlab-org/gitlab-shell/-/jobs/126362702.
Closes #169 (closed)
Edited by Stan Hu