Fix cube.copy() for scalar cubes.

Problem as mentioned https://github.com/SciTools/iris/pull/1403#issuecomment-59939103
```
>>> cube = iris.cube.Cube(0)
>>> cube.copy().data is cube.data
True
>>> 
```

Basically, with cube.copy(**data=None**), the array never got actually copied.
The proposed new code is now actually a bit simpler, from using array copy method.