Ratelimit on multiple threads reset counter

### Summary

Using multiple goroutines my ratelimit-observed on unauthenticated call is reset to 0

### Steps to reproduce

Using this gist https://gist.github.com/Lorac/6cef9747e6e668e99b4c246bfe8d9408 

You gonna have to change one line in the source code 

Comment that line : https://github.com/xanzy/go-gitlab/blob/26ea551e8c159cea42a9f206bc18ae5884d44d0c/gitlab.go#L554 to have unauthenticated calls

### Example Project

Current version of gitlab on live servers

### What is the current *bug* behavior?

Resets the ratelimit to zero, so in theory I could abuse the API by using multiple threads...? You can do the same thing with cURL

### What is the expected *correct* behavior?

The ratelimit should keep incrementing even if I started multiple threads

### Relevant logs and/or screenshots


Very fast output multiple calls...

Ratelimit-Observed:[2] Ratelimit-Observed:[2] Ratelimit-Observed:[2] Ratelimit-Observed:[2]

Output of checks

This bug happens on GitLab.com

gitlab-ce~2278648