Fix NullReferenceException when using BasicHttpBinding and NTLM web proxy authentication

[fondencn]

Fixed Check for SecurityTokenManager initialization for web proxies with NTLM authentication:
When using BasicHttpClient Bindings with additional authentication for enterprise style web proxies, a NullReferenceException was raised because of missing initialization of SecurityTokenManager (resulting member field was NULL).

By extracting the AuthenticationScheme passed by the used binding and using it for deciding whether SecurityTokenManager is needed or not, the NRE is removed and NTLM auth can be used successfully within the HTTP channel factory.

[fondencn]

Team-Internal review needed first

[fondencn]

Created issue #4557 for this PullRequest

[dnfadmin]

All CLA requirements met.

[mconnew]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[fondencn]

@mconnew Will this PR be accepted/merged soon? Not sure about how this works in the dotnet/wcf project... Am I still missing something? There is still a discussion and another PR in issue #4557 with another fix proposal...