fix: wrap custom field titles that contain the delimiter

[greymoth-jp]

When a key is given a custom title via keys: [{ field, title }], the title is written into the header row without passing through wrapFieldValueIfNecessary. Field names (in wrapHeaderFields) and record values both go through that helper, so a title that contains the field delimiter, the wrap delimiter, or a line break leaks into the header and breaks the column layout.

json2csv([{ price: 100, qty: 2 }], {
  keys: [
    { field: 'price', title: 'Price, USD' },
    { field: 'qty', title: 'Qty' },
  ],
});
// header row: Price, USD,Qty   (three columns instead of two)

Reading that output back with csv2json returns {"Price":100," USD":2,"Qty":""} rather than two aligned columns.

RFC 4180 treats the header line as a normal record (section 2.3), and any field containing the delimiter or a double quote must be enclosed in double quotes (sections 2.6 and 2.7). The title replaces a field name in the header, so it needs the same wrapping the field name would have received.

The fix wraps the title in generateCsvHeader with the existing wrapFieldValueIfNecessary helper. Non-titled header fields are already wrapped earlier in wrapHeaderFields, so only the title branch is changed, which avoids double-wrapping.

Added a regression test; the full suite passes.