[REG 5.6.2->5.8.0] MySQL driver fails on slovenian characters

I found what looks like a regression in the MySQL plugin. While I could still normally read/write slovenian characters (for example '蚞ȊŽ') in 5.6.2 version, the same inserts fails with:

+ ErrStr Incorrect string value: '\x8E' for column 'Parameters' at row 1 QMYSQL3: Unable to execute statement INSERT INTO events (Source,Parameters,Start,End,Finished) VALUES (?,?,?,?,0)

I pinned it down to QMySQLDriver::open, where you force the charset to utf8mb4 (didn't check which changeset is responsible) in MySQL server and client are ver 5.5.3 or later.