Multi-value request headers are serialized to comma-separated string

[lo1tuma]

Prerequisites

• [X] I have written a descriptive issue title
• [X] I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.23.2

Plugin version

No response

Node.js version

20.x

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

13.0

Description

The .inject() method allows to inject request headers which values can be a string or an array of strings. When an array is used, it gets serialized to an comma-separated string.

Steps to Reproduce

await fastifyInstance.inject({ method: 'get', url: '/', headers: { foo: 'bar', baz: ['first', 'second'] } });

gives me the following request headers in the route handler:

{
  baz: 'first,second',
   foo: 'bar',
  host: 'localhost:80',
   'user-agent': 'lightMyRequest'
}

Expected Behavior

I would expect to get an array of strings in the route handler as well.