Source: Move around code and change some methods signature.

request_constructor is now a Source method.
update_etags now takes &mut self instead of mut self.
update_etags now uses source setters methods instead of raw fields.
changed the naming of into_feed to to_feed according to rust convention.

hammond-downloader/src/downloader.rs

@@ -239,7 +239,7 @@ mod tests {
         // Copy it's id
         let sid = source.id();
         // Convert Source it into a future Feed and index it
-        let future = source.into_feed(&client, true);
+        let future = source.to_feed(&client, true);
         let feed = core.run(future).unwrap();
         feed.index().unwrap();