Fix #685

httplib.h

@@ -4881,8 +4881,11 @@ inline bool ClientImpl::write_request(Stream &strm, const Request &req,
       auto length = std::to_string(req.content_length);
       headers.emplace("Content-Length", length);
     } else {
+      if (req.method == "POST" || req.method == "PUT" ||
+          req.method == "PATCH") {
         headers.emplace("Content-Length", "0");
       }
+    }
   } else {
     if (!req.has_header("Content-Type")) {
       headers.emplace("Content-Type", "text/plain");