Allow passing body to XHR request

[fvieudrin]

For POST requests for example, it allows to pass a body content to the request as the XHR library can do (option.body) : https://github.com/Raynos/xhr/blob/master/README.md

[heff]

What would you be posting with the XHR lib built into video.js? Internally we only use it for GETs.

Also in master I believe we're still trying to change to raynos/XHR internally. #2321

@dmlap it'd be great to look back into that before 5.0 goes out.

[fvieudrin]

I developed a plugin to log video errors, and I have to post many information in json to my web service. I already use this vjs.xhr() for GET calls, so I want to use the same lib for POST.

[heff]

Thanks for the background, that makes sense. I'd like to get the rest of #2318 figured out before we move forward with this one.

It looks like naugtur/xhr#89 will finish it off, if you want to +1 it.

[gkatsev]

We're now using raynos/xhr, so, I think this is covered by that?

[gkatsev]

Closing this as fixed since we're using the XHR module. If it doesn't suffice, please open an issue. Thanks.