Issue #48 Add support for userinfo_endpoint in OpenIdConnectConfiguratio...

Contains an enhancement to support the userinfo_endpoint value in OIDC Discovery.

Testing was achieved through existing fixture OpenIdConnectConfigurationRetrieverTests. Also, IdentityComparer.AreOpenIdConnectConfigurationEqual() was altered to examine the UserInfoEndpoint property. 

The IdentityComparer modification initially caused the existing test OpenIdConnectConfiguration_Properties to fail, before my changes were applied. I also modified existing test OpenIdConnectConfigurationRetriever_FromText() to compare a loaded configuration against the new OpenIdConfigData.OpenIdConnectConfigurationPing reference instance.

I can't think of any other tests that would add value. Basically, if the constructor's JSON parameter contains a userinfo_endpoint value, then the UserInfoEndpoint property is assigned - this is confirmed by the above two tests. Let me know if you can think of any other tests I should cover.

Thanks
Rich