Update example-reqwest-response

[gerardcl]

Motivation

Related to #2356
Fixes #2429

Solution

Starting to use axum on my projects, and I decided to contribute with an easy fix for the example reqwest-response, since it is a fix similar to what I am using.

The idea of this fix is to avoid the mismatch on http crate from both axum and reqwest crates.
Basically, the main issue is on the headers struct, which I am just remapping to fit into the response builder.

I am not sure if there is a plan to wait for the reqwest to update its dependencies, but in the meantime I think this is a nice and clean fix to avoid having huge commented code snippets.

I hope it makes sense to you!

Thanks!!!

[gerardcl]

@davidpdrsn did update as per your last comment, thanks!
let me know if you need anything else to be updated, i.e. docs, related to this change

[davidpdrsn]

Thanks!