Fix retrieving children of <object> using contents()

[patocallaghan]

Fix for #4384

Summary

Since 0ba8e38, using contents() to get the children of an <object> is not working.

Taking this example HTML

<object type="application/x-shockwave-flash" width="200" height="300" id="penguin">
    <param name="movie" value="flash/penguin.swf">
    <param name="quality" value="high">
    <img src="images/penguin.jpg" width="200" height="300" alt="Penguin">
</object>

I'd expect the following to happen

jQuery(objectHtml).contents().length // => 3

But what actually happens is:

jQuery(objectHtml).contents().length // => 0

Changing the conditional to elem.contentDocument != null handles this case and the function returns the correct children.

Checklist

• All authors have signed the CLA at https://cla.js.foundation/jquery/jquery
• New tests have been added to show the fix or feature works
• Grunt build and unit tests pass locally with these changes
• If needed, a docs issue/PR was created at https://github.com/jquery/api.jquery.com

[mgol]

Thanks for the PR! Looks good to me.

[mgol]

Note to self: remember to cherry-pick this to 3.4-stable.

[timmywil]

This looks good and has enough reviews.

[mgol]

I merged it and the newly added test fails in IE: http://swarm.jquery.org/result/3328701

[mgol]

PR with a fix for IE 11: #4390