Allow custom `Content-Type` header for Nunjucks

[smigles]

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

4.2.0

Plugin version

7.1.2

Node.js version

16.13.0

Operating system

Linux

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

Ubuntu 20.04

Description

Currently, you cannot set a custom Content-Type header if you use Nunjucks template engine. text/html; charset=utf-8 is always sent.

It is similar to #9

Steps to Reproduce

1. Set Nunjucks as the template engine.
2. For example write:

   reply.type('application/xml');
   return reply.view('template.njk', { data });

3. Content-Type response header is text/html; charset=utf-8.

Expected Behavior

Content-Type response header is application/xml; charset=utf-8.

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.