Fix for bug where ExtensionsForBindingSyntax.ToMock<T>() always returns null

There was a problem where the `ExtensionsForBindingSyntax.ToMock<T>()` method always returns null. See the following line:
```C#
return builder as IBindingWhenInNamedWithOrOnSyntax<T>;
``` 
`builder` will never be of type `IBindingWhenInNamedWithOrOnSyntax<T>`.

I have also a method that calls `IBindingWhenInNamedWithOrOnSyntax<T>.ToSingleton()` for convience as this is what one usually wants in a unit test.