Fix combobox GetOptionValue (microsoft#3739)

* Fix combobox GetOptionValue

* Update the current-value

* Fix Unit Test

Author: dvoituron

src/Core/Components/List/FluentCombobox.razor

@@ -14,7 +14,7 @@
                      style="@StyleValue"
                      autocomplete="@Autocomplete.ToAttributeValue()"
                      open=@Open
-                     current-value=@Value
+                     current-value=@GetComboboxContent()
                      placeholder=@Placeholder
                      position=@Position.ToAttributeValue()
                      disabled=@Disabled

src/Core/Components/List/FluentCombobox.razor.cs

@@ -163,11 +163,11 @@ protected override async Task ChangeHandlerAsync(ChangeEventArgs e)
         }
     }
 
-    protected override string? GetOptionValue(TOption? item)
+    private string? GetComboboxContent()
     {
-        if (item != null)
+        if (SelectedOption != null)
         {
-            return OptionText.Invoke(item) ?? OptionValue.Invoke(item) ?? item.ToString();
+            return OptionText.Invoke(SelectedOption) ?? OptionValue.Invoke(SelectedOption) ?? SelectedOption.ToString();
         }
         else
         {

tests/Core/List/FluentComboboxTests.FluentCombobox_ClearSelection.verified.html

@@ -1,5 +1,5 @@
 
 <style>#myComponent::part(listbox) {  z-index: 9995 }</style>
-<fluent-combobox id="xxx" current-value="" position="below" blazor:oncomboboxchange="1" blazor:oncontrolinput="2" blazor:elementreference="">
+<fluent-combobox id="xxx" position="below" blazor:oncomboboxchange="1" blazor:oncontrolinput="2" blazor:elementreference="">
   <fluent-option id="xxx" value="Contoso" blazor:onclick="3" aria-selected="false" blazor:elementreference="">Contoso</fluent-option>
 </fluent-combobox>