util/network/http: Use cargo/1.2.3 user-agent header

... instead of `cargo 1.2.3`.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent#syntax declares that the product and product version are usually separated by a slash. This commit changes the cargo `User-Agent` header to follow that syntax instead of using whitespace for the separator.

Author: Turbo87

src/cargo/util/network/http.rs

@@ -74,7 +74,7 @@ pub fn configure_http_handle(gctx: &GlobalContext, handle: &mut Easy) -> CargoRe
     if let Some(user_agent) = &http.user_agent {
         handle.useragent(user_agent)?;
     } else {
-        handle.useragent(&format!("cargo {}", version()))?;
+        handle.useragent(&format!("cargo/{}", version()))?;
     }
 
     fn to_ssl_version(s: &str) -> CargoResult<SslVersion> {