BUG: nanstd/nanvar gives spurious ComplexWarning when input is real but dtype is complex

When the input array is a real array, but the dtype argument is given to be complex, the nanstd and nanvar functions will give a ComplexWarning, even though the complex part is always zero. This is due to the line:

```
np.subtract(arr, avg, out=arr, casting='unsafe')
```

where `arr` is the original type, but `avg` the new (complex) type, which gives the warning.
