seq() converts pandas DataFrame into Sequence 

```
from functional import seq
from pandas import DataFrame

df = DataFrame({'col1': [1,2,3], 'col2': [4,5,6]})
s = seq([df])
el = s.first()
print(type(el))
```

this code prints: "<class 'functional.pipeline.Sequence'>"
but the expected output is "<class 'pandas.core.frame.DataFrame'>"
