Set a user agent with the version when talking to the registry

This can be used to track what versions of Cargo are being used in the
wild, which can be very useful.

src/cargo/ops/registry.rs

@@ -12,6 +12,7 @@ use term::color::BLACK;
 
 use url::percent_encoding::{percent_encode, QUERY_ENCODE_SET};
 
+use version;
 use core::source::Source;
 use core::{Package, SourceId, Workspace};
 use core::dependency::Kind;
@@ -196,6 +197,7 @@ pub fn http_handle(config: &Config) -> CargoResult<Easy> {
     handle.connect_timeout(Duration::new(30, 0))?;
     handle.low_speed_limit(10 /* bytes per second */)?;
     handle.low_speed_time(Duration::new(30, 0))?;
+    handle.useragent(&version())?;
     if let Some(proxy) = http_proxy(config)? {
         handle.proxy(&proxy)?;
     }