Support property getters with `When..Do` syntax

[dtchepak]

As raised in #449, it would be useful to support property getters in When..Do:

// Current syntax:
sub.When(x => { _ = x.Prop; }).DoNotCallBase(); 
// Proposed:
sub.When(x => x.MyProp).DoNotCallBase();

From @zvirja's comment:

    public static WhenCalled<TSubstitute> When<TSubstitute, TResult>(
            this TSubstitute substitute, Func<TSubstitute, TResult> substituteCall)
            where TSubstitute: class
    {
        return SubstituteExtensions.When(substitute, x => substituteCall(x));
    }

Need to ensure this does not conflict with the existing WhenCalled<T> When<T>(this T substitute, Func<T, Task> substituteCall) overload of this method.

[304NotModified]

Need to ensure this does not conflict with the existing WhenCalled<T> When<T>(this T substitute, Func<T, Task> substituteCall) overload of this method.

I think it does as Task is a class.

Another option is to remove the Func<T, Task> and do the type checking in this one.

Otherwise v6?