add "Make a partial download with HTTP range headers" example

[budziq]

Use reqwest - in conjunction with HTTP range headers to make a partial download of a large file with progress.

cc #84

[sofiageo]

Seems like @jbolila beat me to it :) I didn't make any request comment because i didn't know how far I could go, but it was my first try with rust.

I will still post the code I had so far for any comments

#[macro_use]
extern crate error_chain;
extern crate reqwest;

use std::io::Read;
use reqwest::header::{Headers, Range};

error_chain! {
    foreign_links {
        Io(std::io::Error);
        HttpRequest(reqwest::Error);
        ParseRequest(reqwest::UrlError);
    }
}

fn run() -> Result<()> {
    let url = reqwest::Url::parse("https://rust-lang-nursery.github.io/rust-cookbook/")?;
    let client = reqwest::Client::new()?;

    print!("Making a HEAD request: ");
    let mut req = reqwest::Request::new(reqwest::Method::Head, url.clone());
    let mut res = client.execute(req)?;
    println!("done!");
    let headers= res.headers();
    let supported = headers.get_raw("Accept-Ranges");
    if supported.is_some() {
            let mut headers = Headers::new();
            headers.set(Range::bytes(1, 100));

            println!("Making a GET request: ");
            let mut res = client.get(url.clone())?.headers(headers).send()?;
            let mut body = String::new();
            res.read_to_string(&mut body)?;

            println!("Status: {}", res.status());
            println!("Headers:\n{}", res.headers());
            println!("Body:\n{}", body);
    }

    Ok(())
}

quick_main!(run);

[budziq]

@sofiageo no reason to shy away from claiming an issue, we are not pressed for any deadline here! Just pick another issue and start working on it.

In regard to how this issue should be done, please checkout @jbolila 's PR once it's ready (I'll probably do a review there tomorrow). Your code is quite nice but with few suggestions:

• use convenience method https://docs.rs/reqwest/0.8.0/reqwest/struct.Client.html#method.head instead of reqwest::Request::new(reqwest::Method::Head
• use typed api headers.get instead of headers.get_raw
• no need to create temporary mut headers variable it can be constructed within the ?.headers(..).send call
• we might prefer to read bytes instead of into string (strings must be utf8 and typical binary data would not)
• we would like to read the supported range from the header to request an appropriate range

[budziq]

Merged!