Fixed generated urls not matching webmocks for nil parameters.

[BenjaminSchaaf]

When running the tests, WebMock would break for any API calls with nil parameters. The reason being, that WebMock would generate a slightly different URL to Typhoeus: (I've indicated the difference)

https://openapi.youku.com/v2/videos/show_batch.json?client_id=client-id&ext=&video_ids=123
                                                                           |
https://openapi.youku.com/v2/videos/show_batch.json?client_id=client-id&ext&video_ids=123

Changing the parameters from nil to '' to WebMock fixes this.

[avokhmin]

👍