Multi row copy/paste inserts '\r' on paste for the last selected column

[Marc-Andre-Rivet]

Found while working on Issue #75.

Select at least two rows of cells and copy paste -- the inner html of the cell-value element contains pending '\r' that was inserted by our custom copy logic here:

dash-table/src/dash-table/utils/TableClipboardHelper.ts

Line 22 in 06ac08f

).join('\r\n');

    const value = R.slice(
        R.head(selectedRows) as any,
        R.last(selectedRows) as any + 1,
        dataframe
    ).map(row =>
        R.props(selectedCols, R.props(R.pluck('id', columns) as any, row) as any)
    ).map(row => R.values(row).join('\t')
    ).join('\r\n');

SheetClip provides a stringify function along with the parse function, we should use that instead of our custom logic.

[Marc-Andre-Rivet]

In PR #87

[Marc-Andre-Rivet]

@cldougl Suggest making this no less than a p2 as it introduces invalid data in the dataframe and the workaround is painful (copy/paste single row only)