REGR: pd.unique casts nan to string with other strings

[TomAugspurger]

Code Sample, a copy-pastable example if possible

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

# 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)

[jreback]

hmm, unique is taking a more general path now, I think this is inferred to string (and not object)