Use Result#type instead of setting the header manually when setting content types

src/main/java/net/frozenorb/apiv3/filters/ContentTypeFilter.java

@@ -7,7 +7,7 @@ import spark.Response;
 public final class ContentTypeFilter implements Filter {
 
     public void handle(Request req, Response res) {
-        res.header("Content-Type", "application/json");
+        res.type("application/json");
     }
 
 }