don't add publishedAt offset to list endpoint. Fixes #89

before: 

```ruby
Yt::Collections::Videos.new.where(id: "xKXEr0dZwM4,1OGDzm9YylY", part: 'statistics').map { |v| v.view_count }
 => Yt::Errors::RequestError
# "response_body":{"error":{"errors":[{"domain":"youtube.part","reason":"unknownPart","message":"statistics","locationType":"parameter","location":"part"}],"code":400,"message":"statistics"}}}
```

after:

```ruby
Yt::Collections::Videos.new.where(id: "xKXEr0dZwM4,1OGDzm9YylY", part: 'statistics').map { |v| v.view_count }
 => [28956, 601505]
```