Inconsistent indexing of mixed type indexed Series
Consider the following code:

``` python
  import pandas as pd
  sr = pd.Series(data=[1,2,3,4], index=[1,2, '3', '4'])
  print sr[1]
  print sr[[1,2]]
```

Note that the rows of the Series are both indexed with integers and strings.
This generates the following output:
1
2    2
3    3

The first print statement selects the value correponding to the key 1, i.e. 1
The second print statement should print:
1   1
2    2
However, instead it selects row 1 to 2 and prints the result above.
