add test case for operator=
add test case for operator=

rku/marble,quannt24/marble,oberluz/marble,David-Gil/marble-dev,quannt24/marble,Earthwings/marble,adraghici/marble,rku/marble,probonopd/marble,David-Gil/marble-dev,probonopd/marble,probonopd/marble,adraghici/marble,utkuaydin/marble,AndreiDuma/marble,probonopd/marble,utkuaydin/marble,tucnak/marble,probonopd/marble,tzapzoor/marble,probonopd/marble,tucnak/marble,quannt24/marble,tucnak/marble,David-Gil/marble-dev,quannt24/marble,tzapzoor/marble,Earthwings/marble,tucnak/marble,tzapzoor/marble,Earthwings/marble,adraghici/marble,David-Gil/marble-dev,tzapzoor/marble,tzapzoor/marble,tucnak/marble,quannt24/marble,utkuaydin/marble,oberluz/marble,utkuaydin/marble,quannt24/marble,Earthwings/marble,rku/marble,tucnak/marble,oberluz/marble,tzapzoor/marble,AndreiDuma/marble,adraghici/marble,rku/marble,AndreiDuma/marble,AndreiDuma/marble,rku/marble,quannt24/marble,probonopd/marble,David-Gil/marble-dev,oberluz/marble,adraghici/marble,oberluz/marble,David-Gil/marble-dev,tucnak/marble,Earthwings/marble,tzapzoor/marble,rku/marble,utkuaydin/marble,AndreiDuma/marble,Earthwings/marble,tzapzoor/marble,utkuaydin/marble,AndreiDuma/marble,adraghici/marble,oberluz/marble