fix multiple creds support for 'jwt' and 'key-auth'

[nakardo]

See: #856

Also introduces changes on the credentials.test.js tests to make sure other policies work as intended.

[codecov]

Codecov Report

Merging #899 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #899      +/-   ##
==========================================
+ Coverage   89.07%   89.08%   +0.01%     
==========================================
  Files         136      136              
  Lines        3688     3692       +4     
==========================================
+ Hits         3285     3289       +4     
  Misses        403      403

Impacted Files	Coverage Δ
lib/services/credentials/credential.service.js	86.74% <100%> (+0.29%)	⬆️

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[XVincentX]

Hey,

thanks a lot for taking care of this! The pull request looks good, I just dropped some comments here and there to enhance it a little bit.

Thanks!

[XVincentX]

This now generally feels ok. I need to pull down your branch and make some smoke tests to make sure we do not screw up anything. Good job though, I think this is going to get merged really soon.