Concat floats changes all ints to floats.

```
In [1]: import pandas as pd
In [2]: import numpy as np
In [3]: df = pd.DataFrame([[1, 2], [3, 4]], columns=['A', 'B'], index=['foo', 'bar'])
In [4]: df
Out[4]:
     A  B
foo  1  2
bar  3  4

In [5]: df.append(pd.Series(np.random.rand(2), index=['A', 'B'], name='baz'))
Out[5]:
            A         B
foo  1.000000  2.000000
bar  3.000000  4.000000
baz  0.190326  0.332797

```
#### Problem description
My use case was adding a row that is the result of `bar/foo` to represent it as a percentage.

#### Expected Output
```
Out[5]:
           A        B
foo      1         2
bar      3         4
baz  0.190326  0.332797
```
