DBD::Sponge corrected and lazy default PRECISION 

DBD::Sponge defaults statement handle PRECISION to the length of column NAMEs, which means that a result set's PRECISION can be nonsensically smaller than would actually be required for the data:

    $sponge->prepare("sponges",
                     { NAME =>   ['phylum',   'class'],                # PRECISION defaults to [6,5]
                       rows => [ ['Porifera', 'Hexactinellida'], ] }); # but data would need at least [8,14]

This request changes that default to the columnar maximum lengths of the data, computed only if needed.  Test cases and small POD update.  (Noticed this playing with yet another DBI::Shell alternative.)