feat: Add one Grammatical sugar

[tu6ge]

Examples:

use http::HeaderMap;

fn hello<M: Into<HeaderMap>>(map: M){
    let map: HeaderMap = map.into();
    // more ...
}

// before call it with zero item
use http::HeaderMap;
hello(HeaderMap::new());
// now call it with zero item
hello([]);

[seanmonstar]

Thanks for the PR! However, I don't think having this sugar in the library is what we want. There is the existing FromIterator implementation that helps with this.

[tu6ge]

It suddenly dawned on me, FromIterator is a more abstract From type. Wonderful rust !