[RAILS] Fixed an error in import Rake task

When the task was called without arguments, it failed because `import_model_desc` was nil
[RAILS] Fixed an error, where the `import` Rake task did not use the proxy object

The importing Rake task did not use the `__elasticsearch__` proxy when calling
the `import` method, thus clashing with gems like `activerecord-import` [https://github.com/zdennis/activerecord-import]

Related #164