Fixing melt() when col_level>0 in a multi-index column
Example Input DataFrame -
```
import pandas as pd
x = pd.DataFrame([[0, 1], [2, 3]], columns=[list('AB'), list('CD')])
"""
   A  B
   C  D
0  0  1
1  2  3
"""
x.melt(col_level=1, id_vars=['C'], value_vars=['D'])
```

Current Output -
```
KeyError: 'C'
```

Desired Output -
```
   C variable  value
0  0        D      1
1  2        D      3
```