Fix document.GetOptionsAsync().WithChangedOption potentially throwing

If we both had a document-specific option provided and also tried
to change the value, we'd end up throwing an exception.

src/Workspaces/Core/Portable/Options/OptionServiceFactory.cs

@@ -206,7 +206,7 @@ public DocumentSpecificOptionSet(List<IDocumentOptions> documentOptions, OptionS
 
                 public override OptionSet WithChangedOption(OptionKey optionAndLanguage, object? value)
                 {
-                    return new DocumentSpecificOptionSet(_documentOptions, _underlyingOptions, _values.Add(optionAndLanguage, value));
+                    return new DocumentSpecificOptionSet(_documentOptions, _underlyingOptions, _values.SetItem(optionAndLanguage, value));
                 }
 
                 internal override IEnumerable<OptionKey> GetChangedOptions(OptionSet optionSet)