This isn't technically an issue with this library, however it should be noted that neither Chrome nor Safari currently allow setting the "user-agent" header, both throw Refused to set unsafe header "user-agent" errors.

It might be responsible to detect whether or not the browser allows for setting this header to avoid spurious errors getting thrown.

I'm happy to open a PR if there's a chance it will go in. We'd certainly like that, as currently we're getting this error on every page load.