Fix: undefined method data_accessor_for for StubSessionProxy::Search

**Issue description:**

When I try to use sunspot like this:
```ruby
search = Sunspot.new_search(Post)
search.data_accessor_for(Post).include = %i[comments]
```
Everything works :ok: in `development`, but when I try to run `rspec` I get the following error: 
```
NoMethodError:
    undefined method `data_accessor_for' for #<Sunspot::Rails::StubSessionProxy::Search>
```
(`::Sunspot.session` is set to `StubSessionProxy` obviously)