let the vimeo engine use the returned oembed html instead of constructing an own iframe

[tarzan]

I do not know why, but instead of using the HTML returned by Vimeo's OEmbed API response, Onebox constructs it's own HTML. Resulting in some videos to be rendered as this:

The OEmbed API does return correct HTML, however, so I prefer to use that instead.

(I'm sorry but as the video, that was shown as such, is private I can not share the URI for it)

[tarzan]

Way to reproduce:

1. upload a video to Vimeo
2. set it to private
3. whitelist a domain which is allowed to play the video (don't know if this option is offered on a free account)
4. try a Onebox.preview(URL).to_s and embed the HTML on a page on this whitelisted domain

[arpitjalan]

Thanks for this PR @tarzan!

Can you provide a sample Vimeo private link that is not getting oneboxed by our existing engine? We try to have as much control over the iframe as possible.

Also note that the change in this PR always prefer html over video_id. This is not desirable since public Vimeo videos are getting oneboxed just fine with the video_id that is being returned in oEmbed.