Ensure text/plain & text/html content typs are handled and add missing specs for handled HTTP status codes

Release 8.3.1 to fix version numbering

See merge request gitlab-org/gitlab-shell!241

Release v8.3.0

See merge request gitlab-org/gitlab-shell!240

Includes custom action support for proxying SSH to a Geo secondary

Custom Action support

See merge request gitlab-org/gitlab-shell!215

Release gitlab-shell v8.2.1

See merge request gitlab-org/gitlab-shell!238

GitlabConfig spec was relying upon real config.yml

Proper HTTP status code support for /api/v4/allowed API endpoint

Closes gitlab-ce#51211

See merge request gitlab-org/gitlab-shell!237

* Previously, a 200 (OK) was sent when the user was unauthorised or the project was not found (or the user didn't have access)
* We still treat 401 and 404 as 'success' but we need to explicitly handle them

Release gitlab-shell v8.2.0

Closes #153

See merge request gitlab-org/gitlab-shell!236

Fix handling non 200 response codes

Closes #152

See merge request gitlab-org/gitlab-shell!235

After the cleanup in https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/231
gitlab-shell mishandled the non 200 response codes. This commit removes
another few lines of codes, which fixes this. Also now we test against
this case through mocking.

Fixes https://gitlab.com/gitlab-org/gitlab-shell/issues/152

Cleanup `git-upload-*` and `git-receive-*` related code

Closes gitaly#1300

See merge request gitlab-org/gitlab-shell!232

Given the gitaly-* now proxy the data from the client to the Gitaly
server, the environment variables aren't used. Therefor we don't have to
set them either. Only exception to the rule, is the GITALY_TOKEN.

These changes also remove the `GIT_TRACE` options, introduced by
192e2bd3.

Part of: https://gitlab.com/gitlab-org/gitaly/issues/1300

All shell access goes through Gitaly, so dead code paths exist to
support the legacy way too.

This change mostly removes the dead code from `#process_cmd`.

Remove repo_path from GitlabShell

Closes #135

See merge request gitlab-org/gitlab-shell!231

The internal api returns '/' from gitlab, since
`8fad07383ada021fc995294fd0fe0f77fe37da35` from GitLab CE. To clean up
later, https://gitlab.com/gitlab-org/gitlab-shell/issues/135 was
created.

This change closes that issue, making it possible to remove the field
from the response on GitLab-CE too. Given the Rails app always returns
`/` as the repository_path, the associated checks are basically a noop
too. The tests are updated and at times look a little fishy, but those
are testing code that is to be removed in another MR.

Closes https://gitlab.com/gitlab-org/gitlab-shell/issues/135

Update to RSpec 3

Closes #54

See merge request gitlab-org/gitlab-shell!229

This defines `ROOT_PATH`, which we later stub.