Test default serializer:
obj.to_string(standard)=0.5
Test default serializer with custom userdata:
obj.to_string(userdata)=0.5
Test explicit serializer with custom userdata:
obj.to_string(custom)=test
Test reset serializer:
obj.to_string(reset)=0.5
obj.to_string(default format)=0.52381
obj.to_string(with global format)=x0.524y
obj.to_string(with thread format)=T0.52X
obj.to_string(long thread format)=Ttttttttttttt0.52xxxxxxxxxxxxxxxxxxX
obj.to_string(back to global format)=x0.524y
obj.to_string(back to default format)=0.52381
obj(12.0).to_string(default format)=12.0
obj(12.0).to_string(%.0f)=12
obj(12.0).to_string(%.0g)=1e+01
obj(12.0).to_string(%.1g)=12.0
