REGR: pd.unique casts nan to string with other strings
#### Code Sample, a copy-pastable example if possible

```python
In [1]: import numpy as np; import pandas as pd
In [2]: pd.unique(['a', np.nan, 'c', 'c'])
Out[2]: array(['a', 'nan', 'c'], dtype=object)

In [3]: pd.__version__
Out[3]: '0.20.0rc1'

```
Notice the quotes around `'nan'`. This came up from the seaborn test suite

w#### Expected Output

```python
# 0.19.2
In [1]: import numpy as np; import pandas as pd
In [2]: pd.unique(['a', np.nan, 'c', 'c'])
Out[2]: array(['a', nan, 'c'], dtype=object)
```