fastify-http-proxy not removing prefix

[BlazingAsher]

🐛 Bug Report

When using fastify-http-proxy, the library does not remove the prefix from the request URL before proxying it to the upstream.

To Reproduce

Steps to reproduce the behavior:

fastify.register(require('fastify-http-proxy'), {
    upstream: 'example.com',
    prefix: 'testprefix'
 });

fastify.listen(3005, '0.0.0.0', function (err, address) {
    if (err) {
        fastify.log.error(err)
        process.exit(1)
    }
    fastify.log.info(`server listening on ${address}`)
});

For the request /testprefix/users, the upstream receives /testprefix/users instead of /users.

Expected behavior

The prefix should be removed from the URL before proxying it to the upstream.

Your Environment

• node version: 14
• fastify version: >=3.14.2
• os: Windows

[BlazingAsher]

Looking through the code, it seems line 185 should be fastify.prefix instead of this.prefix.

[mcollina]

makes sense!
Would you like to send a Pull Request to address this issue? Remember to add unit tests.