Result: allow default constructor (#1609)

httplib.h

@@ -959,6 +959,7 @@ std::ostream &operator<<(std::ostream &os, const Error &obj);
 
 class Result {
 public:
+  Result() = default;
   Result(std::unique_ptr<Response> &&res, Error err,
          Headers &&request_headers = Headers{})
       : res_(std::move(res)), err_(err),
@@ -987,7 +988,7 @@ public:
 
 private:
   std::unique_ptr<Response> res_;
-  Error err_;
+  Error err_ = Error::Unknown;
   Headers request_headers_;
 };