gitlab global settings "test connection" fails unconditionally

[a2f0]

Issue

Context

• Gitlab plugin version: 1.4.8 & 1.5.1
• Gitlab version: https://gitlab.com
• Jenkins version: 2.73.3 & 2.60.3
• Job type: N/A

Logs & Traces

Nov 16, 2017 6:30:01 PM FINEST com.dabsquared.gitlabjenkins.gitlab.GitLabClientBuilder
Call GitLab:
HTTP method: HEAD
URL: https://gitlab.com/api/v3/user
Request headers: [
Accept = [application/json],
PRIVATE-TOKEN = [****FILTERED****]
]
Nov 16, 2017 6:30:02 PM FINEST com.dabsquared.gitlabjenkins.gitlab.GitLabClientBuilder
Got response from GitLab:
URL: https://gitlab.com/api/v3/user
Status: 400 Bad Request
Response headers: [
Cache-Control = [no-cache],
Connection = [keep-alive],
Content-Type = [text/html]
]
Response body: 

Problem description

This problem is easily reproducible. I have a feeling it is related Gitlab updating their platform today as my Gitlab connection was working fine but broke suddenly today. The issue can be reproduced by:

1. Do a clean pull of the jenkins docker image
2. Install the gitlab plugin upon first setup
3. Configure an API key credential in https://gitlab.com
4. In Jenkins global setting, configure the credential and https://gitlab.com and click "Test Connection". This will generate an ERROR. The com.dabsquared debug log generated by this failure is above. I get the same results with using the v4 api on the 1.5.1 plugin.

[omehegan]

Maybe related to #608 ? Need @dblessing or someone from GitLab to take a look.

[a2f0]

Thank you for your support @omehegan & @dblessing. Whatever problem this is/was has appeared to go away on its own. I am resolving this issue.