df.info() incorrectly reports memory use for string data?
Doubling the string size does not change the reported memory use:

``` python
In [64]: d=pd.DataFrame(["%06d" %x for x in range(1000000)])
    ...: d.info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 1000000 entries, 0 to 999999
Data columns (total 1 columns):
0    1000000 non-null object
dtypes: object(1)
memory usage: 15.3 MB

In [65]: d=pd.DataFrame(["%012d" %x for x in range(1000000)])
    ...: d.info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 1000000 entries, 0 to 999999
Data columns (total 1 columns):
0    1000000 non-null object
dtypes: object(1)
memory usage: 15.3 MB
```
