[Rust Server] Fix panic handling headers

Created by: richardwhiuk

If we have an API which has multiple auth types, we may panic. This is because in Hyper 0.11, the following code will panic:

use hyper::header::{Authorization, Basic, Bearer, Headers};

fn main() {
    let mut headers = Headers::default();
    let basic = Basic { username: "richard".to_string(), password: None };
    headers.set::<Authorization<Basic>>(Authorization(basic));
    println!("Auth: {:?}", headers.get::<Authorization<Bearer>>());
}

as it mixes up an Authorization<Basic> and Authorization<Bearer> as both have Authorization: as the header name.

This is fixed by using swagger::SafeHeaders added in https://github.com/Metaswitch/swagger-rs/pull/90

This code is no longer relevant once the Hyper 0.12 work is merged, but that is a breaking change, so this is put in for the 4.x branch.

Rust Technical Committee

@frol @farcaller @bjgill

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