to_html for a row-wise empty dataframe should print the header
I'd expect the output of

```
pd.DataFrame(columns=["asdf", "bsdf"]).to_html()
```

to be essentially `<th>asdf</th><th>bsdf</th>` as an html table. This would be a lot more homogeneous. The current output is pretty useless, being basically `repr(df)`.
