Use the enable-if for non-explicit constructors trick that `std::optional` uses to make `std::optional< std::unique_ptr< Base > >` accept `std::unique_ptr< Derived >` implicitly.

This will permit things like this, among others:

```
StatusWith< boost::optional< std::unique_ptr< Base > > >
function()

{   if( condition1() ) return boost::none;   if( condition2() ) return std::make_unique< Derived >();   if( condition3() ) return nullptr;   return Status( ErrorCodes::SomethingFailed, "Something is broken" ); }

```