Allow overriding the default_headers of a client, rather than adding to it

[4JX]

Calling default_headers on a client will add to its internal HeaderMap, rather than replace it.

This is an issue for me, as something like the following:

use reqwest::header::{HeaderMap, ACCEPT, USER_AGENT};

fn main() {
    let mut map = HeaderMap::new();

    map.insert("some_header", "some_value".parse().unwrap());
    map.insert(USER_AGENT, "my_agent".parse().unwrap());
    map.insert(ACCEPT, "text/html".parse().unwrap());

    let client = reqwest::blocking::Client::builder()
        .default_headers(map)
        .build()
        .unwrap();

    let response = client.get("https://tls.peet.ws/api/all").send().unwrap();

    println!("{}", response.text().unwrap());
}

Leaves ACCEPT at the top:

...
"headers": [
  "accept: text/html",
  "some_header: some_value",
  "user-agent: my_agent",
  "host: tls.peet.ws"
]
...

Header ordering is not supposed to matter, but given that the function takes the new map by value, could its signature be changed to the following?

pub fn default_headers(mut self, headers: HeaderMap) -> ClientBuilder {
-   for (key, value) in headers.iter() {
-       self.config.headers.insert(key, value.clone());
-   }
+   self.config.headers = headers;
    self
}

A separate function would also work.