Access Tokens returned but no raw data (on-prem gitlab)

Nobody has claimed this yet.

Successfully logged in using GitHub and google

Version 13.1.4 of gitlab-ce
```
2020-07-17T15:52:32.128 | ERROR | (goth.User) {
RawData: (map[string]interface {}) ,
Provider: (string) (len=6) "gitlab",
Email: (string) "",
Name: (string) "",
FirstName: (string) "",
LastName: (string) "",
NickName: (string) "",
Description: (string) "",
UserID: (string) "",
AvatarURL: (string) "",
Location: (string) "",
AccessToken: (string) (len=64) "ae4dc7c22470c8de4031ff3177c4029365fca77acf2fe6b59786d038636fa889",
AccessTokenSecret: (string) "",
RefreshToken: (string) (len=64) "6eaaad2f48b3e3777e151b6ad8f4c2e1bd0d1a15d497520830d315c22e35be53",
ExpiresAt: (time.Time) 0001-01-01 00:00:00 +0000 UTC,
IDToken: (string) ""
}
2020-07-17T15:52:32.128 | ERROR | invalid character '<' looking for beginning of value
```

Start with the GitLab provider and reproduce the login against GitLab CE 13.1.4, comparing its response with the successful GitHub and Google logins. Trace the response parsing that leaves RawData and user fields empty and produces the invalid-character error; done means the GitLab login returns populated user data without a parsing error.

Written by the indexing model from the issue text.