Make sub claim value single value only

[cicnavi]

In custom scopes, one can also use standard claims. Custom scopes have an option to allow a claim to have multiple values. Standard claims have predefined format, as per https://openid.net/specs/openid-connect-core-1_0.html#StandardClaims.
OIDC module expects certain claims to have single value (for example, sub claim). Setting it to multi value brakes the app. This PR solves that.

Note: this PR only solves the sub claim, while the other user claims will be resolved in v7 branch, since this can potentially be a breaking change (one can set standard claim as multi value in custom scope, and the OIDC module wouldn't mind, as it doesn't reference them in code).

Closes #308

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.62%. Comparing base (0080cf2) to head (0777e28).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #309      +/-   ##
============================================
+ Coverage     53.53%   53.62%   +0.08%     
- Complexity     1488     1489       +1     
============================================
  Files           141      141              
  Lines          6459     6460       +1     
============================================
+ Hits           3458     3464       +6     
+ Misses         3001     2996       -5     

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[mrvanes]

Tested and fixes #308