CURLOPT_COOKIEFILE and Easy::cookie_file

Hello all,

The libcurl doc for [CURLOPT_COOKIEFILE](https://curl.se/libcurl/c/CURLOPT_COOKIEFILE.html) states that:

> Setting this option to NULL disables the cookie engine and clears the list of files to read cookies from.

I've a use case where an `Easy` handle configures a certain cookie file, and then should be set to null for subsequent request (reusing the same handle).

The actual signature of the `Easy` cookie file method is:

Would the curl crate project accepts a PR that changes the signature to pass a NULL to CURLOPT_COOKIEFILE (changing the `P` parameter to `Option

`):

```rust
pub fn cookie_file>(&mut self, file: Option

) -> Result<(), Error> {
```

The same kind of signature that other methods have:

https://github.com/alexcrichton/curl-rust/blob/14d51cf1f364351456dfd9ab89521242873fb95d/src/easy/handle.rs#L168

I'm asking this question because I've the impression (certainly not real and not a critique at all) that the crate is now only accepting updates to the libcurl binding PR kind.

Thanks a lot for any feedback,