fix: prevent multiple location headers on redirect (#3298)

[veigaribo]

The problem turned out to be specific to 0.7 Actix, which I did not expect.

This solution is what was proposed by @gbj in #3298, although I did not make the new code part of extend_response because extend_response is also called in handle_static_route, and I don't think it needs this (maybe I'm wrong?).

I believe a better solution would actually be to match the Axum behavior, that uses http::HeaderMap::extend: For merging map A with map B, if map B contains a key that is present in A, all values with such key in A are removed and all values with that key in B are added. That would at least be more consistent.

I also still don't understand why both server_fn::ServerFn::run_on_server and leptos_actix::handle_server_fns_with_context need to handle redirection to Referer, but I assume there is a reason.

Took the oportunity to fix the surrounding comments too, to the best of my ability.

[benwis]

I think the problem with this one is that headers like Set-Cookie are set multiple times and should not be overwritten this way

[veigaribo]

I think the problem with this one is that headers like Set-Cookie are set multiple times and should not be overwritten this way

I think this solution is pretty hacky, but it should not affect any header other than Location 🤔 because that's what we call .remove() on

[benwis]

LGTM, seems ideal