Have QHttpNetworkReply try to interpret the HTTP reason phrase as UTF-8 ...

...first.

Nowadays, despite what the RFCs say, HTTP reason phrases that aren't ASCII
are far more likely to be UTF-8 than ISO-8859-1.  Qt blindly interprets
them as ISO-8859-1, which causes them to get double-coded in logs and such.
Unfortunately, this can only be fixed deep in the guts of Qt.

https://github.com/ariya/phantomjs/issues/12758