Modify upsert to return an entity if a key has been found

[dimitriadamou]

This is a very simple change where it will search the values of $wheres in Appsero\LaravelDatastore\Query\Builder if a key of instance type \Google\Cloud\Datastore\Key is found, we will then return the Entity from Google Datastore then apply the mutations via values.

This also allows for Laravel Models to be updated via $model->save();

E.g.

   $myModel = MyModel::where("abc", $value)->first();
   $myModel->def = "newValue";
   $myModel->save();