merge of sparse dataframe fails
This works fine:

```
df = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD'))
df2 = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD'))
df.merge(df2, how="left", on="A")
```

This fails with "TypeError: type object argument after \* must be a sequence, not map"

```
df = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD')).to_sparse()
df2 = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD')).to_sparse()
df.merge(df2, how="left", on="A")
```
