Copying HTML table to spreadsheet software loses top left cell
Right now, if you copy the HTML table representation of a `DataFrame` from Jupyter to Google Sheets or Open office Calc, you lose the top left empty cell (and the top left cell gets the first column name, which is incorrect).

Inserting an `&nbsp;` into that cell prevents this from happening. Would a PR be accepted which did this?

One way of achieving this is to set [`BLANK_VALUE`](https://github.com/pydata/pandas/blob/564166529071d33844712ab6038f6a237ddcb8f1/pandas/core/style.py#L174) to `&nbsp;`. In Google documents then, the pasted cell is present but empty. In open office, the resulting cell has a single space in it (which seems OK).

I don't know what the wider implications of setting `BLANK_VALUE` are - I just want to set the upper left portion of the table so that the heading is correctly aligned when I copy the table. So maybe there is a better way?
