Bugfix: Regression when filling struct from tuple

Here's a bugfix for functionality which regressed from 1.6 -> 1.7, calling x.fill() with a tuple. Should have filed this PR sooner, sorry!

```
>>> import numpy as np
>>> np.__version__
'1.6.2'
>>> x = np.zeros(2, dtype=[('a', 'f8'), ('b', 'i4')])
>>> x.fill((3.5, -2))
>>> x
array([(3.5, -2), (3.5, -2)],
      dtype=[('a', '<f8'), ('b', '<i4')])
      
>>> import numpy as np
>>> np.__version__
'1.7.1'
>>> x = np.zeros(2, dtype=[('a', 'f8'), ('b', 'i4')])
>>> x.fill((3.5, -2))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Input object to FillWithScalar is not a scalar
```