Change syntax for assigning new row values in add_row()

This fixes a problem that comes from the following numpy bug when assigning
to a row that has object-type fields:
```
>>> a = np.array([('1',)], dtype=[('x', 'O')])
>>> a[0] = a[0]
ERROR: ValueError: Setting void-array with object members using buffer. [IPython.core.interactiveshell]
```
Merge pull request #1403 from taldcroft/fix-1403

Fix problem adding a row to a table with object-type field(s)