=head2 $csv->header ($fh)

Return the CSV header and set C<sep_char>.

  my @hdr = $csv->header ($fh);
  my @hdr = $csv->header ($fh, [ ";", ",", "|", "\t" ]);

Assuming that the file opened for parsing has a header, and the header
does not contain problematic characters like embedded newlines, read
the first line from the open handle, auto-detect whether the header
separates the column names with a character from the allowed separator
list. That list defaults to C<[ ";", "," ]> and can be overruled with
an optional second argument. If any of the allowed separators matches
(checks are done in order), set C<sep_char> to that sequence for the
current CSV_XS instance and use it to parse the first line and return
it as an array where all fields are mapped to lower case:

 my $csv = Text::CSV_XS->new ({ binary => 1, auto_diag => 1 });
 open my $fh, "<:encoding(iso-8859-1)", "file.csv";
 my @hdr = $csv->header ($fh) or die "file.csv has no header line\n";
 # $csv now has the correct sep_char
 while (my $row = $csv->getline ($fh)) {
     ...
     }