Fix AntiforgeryMiddlewareTest for non-US date formats

[martincostello]

Fix AntiforgeryMiddlewareTest for non-US date formats

Fix tests failing with HTTP 400 if machine uses a non-US date format.

Description

Use CultureInfo.CurrentCulture to match MVC's default DateTime binding behaviour to fix tests when running on a machine that doesn't use invariant/en-US date formatting, such as en-GB.

Otherwise the line below throws when binding the posted form as the format is invariant but MVC tries to bind it with the current culture.

aspnetcore/src/Mvc/Mvc.Core/src/ModelBinding/Binders/DateTimeModelBinder.cs

Line 67 in dad78b3

model = DateTime.Parse(value, valueProviderResult.Culture, _supportedStyles);

I figured this was the simpler change than reconfiguring MVC in all the tests to use invariant binding by default.

See #55595 (comment).

[captainsafia]

LGTM!

I took a look at the stacktrace for this issue and it looks like prefixing the field names in the tests with __Invariant might also do the trick but this feels more straightforward.