Cubes with time units lose calendar information

[djkirkham]

Probably not a common use case, but still:

>>> import iris
>>> from cf_units import Unit
>>> u = Unit('days since 1970-01-01', calendar='360_day')
>>> cube = iris.cube.Cube([1], standard_name='time', units=u)
>>> iris.save(cube, 'my.nc')

$ ncdump -h my.nc
netcdf my {
dimensions:
	dim0 = 1 ;
variables:
	int64 time(dim0) ;
		time:standard_name = "time" ;
		time:units = "days since 1970-01-01" ;

// global attributes:
		:Conventions = "CF-1.5" ;
}