Users shouldn't have to parameterize their code unless they need to for tests

src/registration.rs

@@ -13,7 +13,7 @@ const DEFAULT_REDIRECT_URI: &'static str = "urn:ietf:wg:oauth:2.0:oob";
 
 /// Handles registering your mastodon app to your instance. It is recommended
 /// you cache your data struct to avoid registering on every run.
-pub struct Registration<'a, H: HttpSend> {
+pub struct Registration<'a, H: HttpSend = HttpSender> {
     base: String,
     client: Client,
     app_builder: AppBuilder<'a>,