sort_values bug while by both list and other values
first, I know the strings in column `b` can be used directly for sorting. My function `aFucntion` seems to be so stupid, because it is just a simplified one to present the problem. In my real situation, it actually produces a list of some English Characters, which then be used to sort words in my native language in a customed way.

Then, I know LIST is not hashable

```python
# coding = utf-8
import pandas as pd

def aFucntion(var):
    return list(var)

a = {'a': [1, 2],
     'b': ['Python', 'Java']
     }

df =pd.DataFrame(a)

print(df)
print('\n')


df['fun'] = df['b'].map(aFucntion)


df1 = df.sort_values(['b']) # this works
print(df1)
print('\n')

df2 = df.sort_values(['fun'])  # this works
print(df2)
print('\n')
```
<pre>
df3 = df.sort_values(['b', 'fun'])  # <b>this yields `TypeError: unhashable type: 'list'`</b>
# but why df.sort_values(['fun'])  works?
</pre>
```python
print(df3)
print('\n')

df['fun'] = df['b'].map(lambda e: tuple(aFucntion(e)))
df4 = df.sort_values(['b', 'fun'])  # this works
print(df4)
print('\n')
```