Verifying partial doubles doesn't work with private methods

I'm just upgrading to RSpec 3, and I have set

```ruby
RSpec.configure do |config|
  config.mock_with :rspec do |mocks|
    mocks.verify_partial_doubles = true
  end
end
```

And in one of my tests I'm stubbing a private method:

```ruby
allow(object).to receive(:some_method)
```

However, I get the error:

```
#<MyObject:0x007ff7cb8d9170> does not implement:
  some_method
```