Cors headers are not properly set.

[AdriVanHoudt]

The problem is that atm we take over response handling (by returning h.abandon) which also means headers handling.
Doing cors handling ourselves would be a bit much, it would be nicer if we could just let hapi do that.

The proposed solution is to properly return/handle the transform streams we use back to hapi and then continue the lifecycle by returning h.continue.

For csv (works):

• Set csvStream._readableState.objectMode = false; and don't pipe res
  • Quoting @kanongil (who helped a lot with these initial findings 🙏)

  It works because the implementation doesn't actually use objectMode for the output, and this is a way to change the mode for only that end

  • I logged an issue on fast-csv to actually fix this [BUG] readable stream part of CsvFormatterStream is in objectMode C2FO/fast-csv#319
• Set the headers on a headers property of the csvStream so that hapi will pick them up
• Return csvStream

For Excel (not tested):

• Pass a duplex stream to WorkbookWriter that has its readable side not in object mode
• Set the headers on a headers property of the duplex stream so that hapi will pick them up
• Pass the duplex stream to hapi

There is branch with tests and a working csv version here: https://github.com/Salesflare/hapi-csv/tree/cors-and-lifecycle-fix