support onLoad synthetic event attribute on 'object' tags

[ara4n]

It seems like a bug that onLoad attributes are ignored on 'object' tags - this fixes it by cargoculting the 'iframe' tag example. This is useful for telling when SVGs have loaded when SVGs are embedded as objects (such that the SVG can be dynamically edited from JS).

[syranide]

I'm curious; should this apply to <embed> as well perhaps? What about the error-event?

[ara4n]

yup, <embed> should probably also fire onLoad, and yup, they should probably also both fire onError too (although from looking at the event inspector in Chrome, it seemed that React had already installed an error listener for <object>). I guess I wanted to check that this is the right approach before extending it to too many other missing tags & events...

[facebook-github-bot]

@ara4n updated the pull request.

[zpao]

I've been playing around with this a bit and I can't get error events out of object elements, nor can I get load (or error) events out of embeds. Also, gets a bit tricky since the load event doesn't even fire consistently (can get it for SVGs but not others). I think this is probably fine pretty much as is. And if we need to we can always follow up. I'll give it a bit of a closer look shortly and merge tonight. Thanks (and sorry for the delay!)