Fix Response::content_length_ not reflecting body size in Logger (Fix #2488)

Server::apply_ranges computed the correct Content-Length header for
body-based responses but never updated content_length_, so the
Logger callback always saw 0. Set content_length_ to the final body
size (post-range/post-compression) alongside the header.

httplib.h

@@ -12323,8 +12323,8 @@ inline void Server::apply_ranges(const Request &req, Response &res,
       }
     }
 
-    auto length = std::to_string(res.body.size());
-    res.set_header("Content-Length", length);
+    res.content_length_ = res.body.size();
+    res.set_header("Content-Length", std::to_string(res.content_length_));
   }
 }