add_response_header will override existing Set-Cookie headers

**Problem**

In fullstack-core/src/streaming.rs, the add_response_header mehod uses insert method to add headers to the response. However, insert method replaces any existing header with the same name, which breaks multi-valued headers like Set-Cookie.
https://github.com/DioxusLabs/dioxus/blob/4b1b60a487b682f62305342c29ece0553d0da8d8/packages/fullstack-core/src/streaming.rs#L196-L205

**Steps To Reproduce**

Steps to reproduce the behavior:

- In a fullstack Dioxus app, call at server function
```rust
ctx.add_response_header(
http::header::SET_COOKIE,
http::HeaderValue::from_str(
"name=example_1",
)
.unwrap(),
);
ctx.add_response_header(
http::header::SET_COOKIE,
http::HeaderValue::from_str(
"name=example_2",
)
.unwrap(),
);
```
- Only last set Set-Cookie header ("name=example_2") appears in the response.

**Expected behavior**

At least regarding the Set-Cookie response header, I believe it's preferable to be able to set and return multiple values.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Set-Cookie

https://datatracker.ietf.org/doc/html/rfc6265#section-3

**Screenshots**

N/A

**Environment:**

- Dioxus version: v0.7.9
- Rust version:
- OS info: Windows 11
- App platform: fullstack web

**Questionnaire**

I would like to fix and I have a solution.

In the add_response_header method, I think modify it to treat Set-Cookie headers as a special case by replacing the insert method with the append method when the argument contains a Set-Cookie header.

```rust
/// Add a header to the response. This will be sent to the client when the response is committed.
/// Set-Cookie headers will be appended to any existing Set-Cookie headers (compliance with RFC 6265),
/// while all other headers will overwrite any existing header with the same name.
pub fn add_response_header(
&self,
key: impl Into,
value: impl Into,
) {
let mut lock = self.lock.write();
if let Some(headers) = lock.response_headers.as_mut() {
let key_into = key.into();
if key_into == http::header::SET_COOKIE {
headers.append(key_into, value.into());
} else {
headers.insert(key_into, value.into());
}
}
}
```