Http window authentication ignore explicitly setting SPN

[hongdai]

It's a no OP now. We will need to document it.

[zhenlan]

This is because of dotnet/corefx#4502.

[roncain]

It looks like https://github.com/dotnet/corefx/issues/4502 is assigned to 'future' so this is probably not going to be available in 1.1.

@hongdai -- how do we repro this failure? Is there an existing test that fails for this reason, or should we write a new one?