mapping with category data does not work
```
import pandas as pd
a = pd.Series([1, 2, 3, 4])
b = pd.Series(["even", "odd", "even", "odd"], dtype="category")
c = pd.Series(["even", "odd", "even", "odd"])
​
print("-- map by category")
try:
    print(a.map(b))
except AttributeError as e:
    print(e)
​
print("-- map by string")
print(a.map(c))
```

has the following output:

> -- map by category
> 'Categorical' object has no attribute 'flags'
> -- map by string
> 0     odd
> 1    even
> 2     odd
> 3     NaN
> dtype: object
