BUG/ENH: dtype change on enlargement
http://stackoverflow.com/questions/22044766/adding-row-to-pandas-dataframe-changes-dtype/22045011#22045011

A bit non-trival (maybe coerce back after) if possible?

maybe just change this to an actual append?

```
In [1]: df = DataFrame({'a' : range(10)}, dtype='i4')

In [2]: df
Out[2]: 
   a
0  0
1  1
2  2
3  3
4  4
5  5
6  6
7  7
8  8
9  9

[10 rows x 1 columns]

In [3]: df.dtypes
Out[3]: 
a    int32
dtype: object

In [4]: df.loc[10] = 99

In [5]: df
Out[5]: 
     a
0    0
1    1
2    2
3    3
4    4
5    5
6    6
7    7
8    8
9    9
10  99

[11 rows x 1 columns]

In [6]: df.dtypes
Out[6]: 
a    float64
dtype: object

```
