impl Default for HttpConfig, so field isn't required in config file

these changes allow omitting the http field of the top-level config,
relying on the defaults instead. previously, not having an [http] section
in the config.toml would fail to parse.

src/main.rs

@@ -194,6 +194,7 @@ pub struct Config {
     pub output: OutputConfig,
     /// Download settings
     #[clap(flatten)]
+    #[serde(default)]
     pub http: HttpConfig,
     /// Specify configuration values using the provided TOML file, instead of
     /// via command line flags. The values in the config file will override
@@ -274,6 +275,16 @@ impl std::fmt::Debug for TargetRegistryConfig {
     }
 }
 
+impl Default for HttpConfig {
+    fn default() -> Self {
+        Self {
+            user_agent: default_user_agent(),
+            requests_per_second: default_requests_per_second(),
+            max_concurrent_requests: default_max_concurrent_requests(),
+        }
+    }
+}
+
 impl std::fmt::Debug for HttpConfig {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         f.debug_struct("Config")