Setting xhr.responseType causes an exception in Firefox

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Current behaviour

#562 changed the way xhr.responseType is set. This line will throw an exception in Firefox, but it only seems to happen when run from a web worker.

InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable

Steps to reproduce (if the current behaviour is a bug)

See socketio/socket.io-client#1149 for a test case

Expected behaviour

Setting xhr.responseType shouldn't cause an error

Setup

• OS: Windows
• browser: Firefox 54.0.1
• engine.io version: 3.1.1

Other information (e.g. stacktraces, related issues, suggestions how to fix)

[darrachequesne]

It seems that's similar to the issue in react native (facebook/react-native#13991).

Also, it's working on Ubuntu with Firefox 54.0.1.

(and similar to #573)

This definitely seems like a Firefox issue. Filed a bug. https://bugzilla.mozilla.org/show_bug.cgi?id=1387075