PostgreSQL and NaN insertion doesn't work

Inserting a value that's not a number into a double percision field in PosgreSQL using QSqlQuery::prepare() doesn't work. Qt tries to execute the following query

insert into table (field) values (nan)

while the query should look like

insert into table (field) values ('nan')

note the quotes around 'nan'.

autotest: tst_QSqlQuery::task_233829()