Function tabular_payload 

pub fn tabular_payload(
    df: &DataFrame,
    format: CopyFormat,
    header: bool,
    html: bool,
) -> Result<Payload, String>

The payload for a tabular copy: the chosen format as text, with the HTML flavor beside a TSV or CSV copy when html asks for it (the destination can offer it). A Markdown copy is the Markdown itself — pasting rich HTML where Markdown was asked for would defeat the choice. List and struct cells are JSON in every format, as in a CSV export.