[FluentWizard] Fix ValueChanged never trigerred in FluentWizard

To fix #1534 detected by @ejalbertinfoej (Thanks).

Issue

Using the sample demo code, we have this code:

@code
{
    int Value = 0;
}

<FluentWizard Id="customized-wizard"
              @bind-Value="@Value"
              StepTitleHiddenWhen="@GridItemHidden.XsAndDown"
              Height="300px">

When clicking Next button in the wizard, the Value stays at 0. The ValueChanged callback method is never called.

Solution

1. Adding the fix.
2. Adding Unit Test to verify this case.