BUG: SparseSeries from dict inconsistency
#### Code Sample, a copy-pastable example if possible
Two (somewhat related) issues:
```python
>>> pd.Series({1: 1})
1    1
dtype: int64

>>> pd.SparseSeries({1: 1})
---------------------------------------------------------------------------
AttributeError: 'dict' object has no attribute 'index'
```

```py
>>> pd.Series({1: 1}, index=[0, 1, 2])
0    NaN
1    1.0
2    NaN
dtype: float64

>>> pd.SparseSeries({1: 1.}, index=[0, 1, 2])
0    1.0
1     
2     
dtype: float64
BlockIndex
Block locations: array([0], dtype=int32)
Block lengths: array([1], dtype=int32)
```
#### Problem description
SparseSeries initialization from dict acts too differently from Series'.

#### Expected Output
```py
>>> pd.SparseSeries({1: 1})
1    1.0
dtype: float64
BlockIndex
Block locations: array([0], dtype=int32)
Block lengths: array([1], dtype=int32)

>>> pd.SparseSeries({1: 1.}, index=[0, 1, 2])
0    NaN 
1    1.0 
2    NaN
dtype: float64
BlockIndex
Block locations: array([1], dtype=int32)
Block lengths: array([1], dtype=int32)
```

#### Output of ``pd.show_versions()``

<details>
pandas 0.21.0.dev+223.ga9421af1a
</details>