astropy.io.ascii.write ignores fill_values

The following syntax is almost straight from the documentation. In this case I am not sure if I do something wrong (if you tell me what, I'll try to see where the documentation can be improved) or this is a bug.
```
In [1]: import astropy

In [2]: import astropy.table

In [3]: import astropy.io

In [4]: import astropy.io.ascii

In [5]: data2 = {'x': [1, 2, 3], 'y': [4, 5.2, 6.1], 'z': ['hello', 'world', '!!!']}

In [6]: tab = astropy.table.Table(data2)

In [7]: astropy.io.ascii.write(tab, sys.stdout)
y x z
4.0 1 hello
5.2 2 world
6.1 3 !!!

In [8]: astropy.io.ascii.write(tab, sys.stdout, fill_values = [('!!!','foo'), ('1', 'bar')])
y x z
4.0 1 hello
5.2 2 world
6.1 3 !!!
```

Clearly, the output with and without `fill_values` is identical.