allow user agent to mention Go.

R=r
http://go/go-review/1024046

src/pkg/http/request.go

@@ -120,8 +120,7 @@ func valueOrDefault(value, def string) string {
 	return def;
 }
 
-// TODO(rsc): Change default UserAgent before open-source release.
-const defaultUserAgent = "http.Client"
+const defaultUserAgent = "Go http package"
 
 // Write writes an HTTP/1.1 request -- header and body -- in wire format.
 // This method consults the following fields of req: