Support masked attribute for CI Variables

[PhilippeVienne]

Support the masked attribute for CI Variables as described in https://docs.gitlab.com/ee/api/project_level_variables.html#create-variable

Working on PR

[gmessner]

@PhilippeVienne
This has been released in 4.11.9. In the process of the release, I found another problem with the GitLab documentation, the masked field is never returned in the JSON from the server, so will always be null. I have changed the integration tests accordingly.

Please verify and close when you have time.

[gmessner]

@PhilippeVienne
Just wondering if you could verify and close this issue?

[gmessner]

@PhilippeVienne
Since the fix has been released I am closing this issue. If you find any problems feel free to re-open.