BUG: crosstab not working for integer types as columns
From the mailing list: https://groups.google.com/forum/?fromgroups#!topic/pydata/fpYfwkZvdxI

```
In [37]: tips = pd.read_csv('https://raw.github.com/pydata/pandas/master/pandas/tests/data/tips.csv')

In [38]: pd.crosstab(tips.day, tips.size)
Out[38]:
col_0  1708
day
Fri      19
Sat      87
Sun      76
Thur     62

In [39]: pd.__version__
Out[39]: '0.16.2'
```

while it should be:

```
In [3]: pd.crosstab(tips.day, tips.size)
Out[3]:
size  1   2   3   4  5  6
day
Fri   1  16   1   1  0  0
Sat   2  53  18  13  1  0
Sun   0  39  15  18  3  1
Thur  1  48   4   5  1  3

In [4]: pd.__version__
Out[4]: '0.14.1'
```

But strange thing is, I can't reproduce it with simple example:

```
In [43]: df = pd.DataFrame({'a':list('abcabcabcab'), 'b':[1,1,1,1,2,3,2,2,3,3,1]
})

In [44]: pd.crosstab(df['a'], df['b'])
Out[44]:
b  1  2  3
a
a  2  1  1
b  2  2  0
c  1  0  2
```
