BUG: fill_value is ignored when reindexing empty series
Here's a snippet depicting the, probably, buggy behaviour:

``` python
>>> import pandas as pd
>>> s = pd.Series()
>>> s.reindex([1,2,3])
1   NaN
2   NaN
3   NaN
dtype: float64
>>> s.reindex([1,2,3], fill_value=0.0)
1   NaN
2   NaN
3   NaN
dtype: float64
>>> pd.version.version
'0.12.0.dev-d7c6eb1'
```

I'd expect the last output to contain `0.0` values instead of `NaN`.
