cdms2 axis cannot have dictionary as attribute values

```python
import cdms2
import MV2

a = MV2.ones((12,12))
ax = cdms2.createAxis(MV2.arange(12))
ax.someattribute = {1:"fg"}

a.setAxis(0,ax)

with cdms2.open("bad.nc","w") as f:
    f.write(a)
```
