Avoid use of Object.keys() in cleanData()

[hiroshi]

Object.keys() is still not supported by some old browsers.  http://kangax.github.io/es5-compat-table/

Also for in is used to iterate event types in other places (i.e. jQuery.event.remove).

for ( type in events ) {
	jQuery.event.remove( elem, type + types[ t ], handler, selector, true );
}

I think we should avoid use of Object.keys() in terms of followings.

• Browser compatibility
• Code consistency

[rwaldron]

Object.keys() is still not supported by some old browsers.

This is not a relevant "issue" for jQuery 2.x, which only supports browsers that _will_ have Object.keys

[dmethvin]

I think the consistency issue is a better argument, since we don't use Object.keys anywhere else. However, the good thing about it would be that potentially we could survive augmentation of Object.prototype if we used this everywhere, since it only enumerates own properties? The for/in makes the code a bit shorter and simpler, I haven't looked to see the impact on gzip though.

[timmywil]

While this is not high priority, I think I would prefer to use the for-in loop to save a few bytes. That is, if the argument is for consistency, the bytes would add up if we used Object.keys ubiquitously. Supporting Object.prototype augmentation is not something we've pursued historically and I don't think we would want to make the necessary changes to the 1.x branch to keep it consistent with 2.x if we wanted to start.

[hiroshi]

I added some analysis. Performance and file size seem better.  https://github.com/jquery/jquery/pull/1472

[Michał Gołębiowski]

Manipulation: don't use Object.keys for consistency

Fixes #14659

Changeset: a96d5bed58f2b30f97d6dd2f5691cd890f62b75f