this._unset is undefined in the browser

[don-smith]

I haven't chased this down completely, but I don't think https://github.com/visionmedia/superagent/blob/master/src/node/index.js#L134 is being invoked in the browser, such as in a webpack bundle. Which means _unset is not defined before https://github.com/visionmedia/superagent/blob/master/src/request-base.js#L367 is called, which is where I'm seeing this error happen. This line was added a couple of hours ago as part of version 5.3.0.

[don-smith]

@dobesv and @niftylettuce I think this is going to affect anyone setting headers in the browser.

[niftylettuce]

I deprecated v5.3.0 in the interim. Let me rollback that commit and publish new.

[niftylettuce]

v5.3.1 published with the fix, in the interim I deprecated v5.3.0 in npm and also commented on #1560 about this

https://github.com/visionmedia/superagent/releases/tag/v5.3.1