From f1ce173283a06197cfde80e9117ba1aa0ed0dd02 Mon Sep 17 00:00:00 2001
From: Joey <jyapayne@gmail.com>
Date: Tue, 20 Apr 2021 23:42:04 -0600
Subject: [PATCH] Fix Httpclient headers from being modified accidentally
 (#17808)

---
 lib/pure/httpclient.nim | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/lib/pure/httpclient.nim b/lib/pure/httpclient.nim
index 14bcfd2fb8..b60894103a 100644
--- a/lib/pure/httpclient.nim
+++ b/lib/pure/httpclient.nim
@@ -962,12 +962,15 @@ proc format(client: HttpClient | AsyncHttpClient,
 
 proc override(fallback, override: HttpHeaders): HttpHeaders =
   # Right-biased map union for `HttpHeaders`
-  if override.isNil:
-    return fallback
 
   result = newHttpHeaders()
   # Copy by value
   result.table[] = fallback.table[]
+
+  if override.isNil:
+    # Return the copy of fallback so it does not get modified
+    return result
+
   for k, vs in override.table:
     result[k] = vs