Fix the broken scheme-less GetTokenAsync overload

[kevinchalet]

The GetTokenAsync() overload that doesn't require a scheme is convenient... but broken, as it tries to call the other overload with a null scheme, which results in an ArgumentNullException being thrown at runtime:

https://github.com/aspnet/HttpAbstractions/blob/dev/src/Microsoft.AspNetCore.Authentication.Abstractions/TokenExtensions.cs#L135

/cc @HaoK @Tratcher

[HaoK]

Yeah, I should add a test for this one :)

[HaoK]

3085f01

[kevinchalet]

Thanks! 👍