Add beginWrite for "flushing" headers

Co-authored-by: notaphplover <notaphplover@users.noreply.github.com>

Author: uNetworkingAB

src/HttpResponse.h

@@ -1,5 +1,5 @@
 /*
- * Authored by Alex Hultman, 2018-2025.
+ * Authored by Alex Hultman, 2018-2026.
  * Intellectual property of third-party.
 
  * Licensed under the Apache License, Version 2.0 (the "License");
@@ -427,6 +427,25 @@ struct HttpResponse : public AsyncSocket<SSL> {
         return this;
     }
 
+    /* Begin writing the response body. Useful for chunked encodings whose first chunk is not yet known */
+    void beginWrite() {
+        /* Write status if not already done */
+        writeStatus(HTTP_200_OK);
+
+        HttpResponseData<SSL> *httpResponseData = getHttpResponseData();
+
+        if (!(httpResponseData->state & HttpResponseData<SSL>::HTTP_WRITE_CALLED)) {
+            /* Write mark on first call to write */
+            writeMark();
+
+            writeHeader("Transfer-Encoding", "chunked");
+            httpResponseData->state |= HttpResponseData<SSL>::HTTP_WRITE_CALLED;
+
+            /* Start of the body */
+            Super::write("\r\n", 2);
+        }
+    }
+
     /* End without a body (no content-length) or end with a spoofed content-length. */
     void endWithoutBody(std::optional<size_t> reportedContentLength = std::nullopt, bool closeConnection = false) {
         if (reportedContentLength.has_value()) {