[System] HttpWebRequest::Headers setter was only setting local variable. Fixes #42408

mcs/class/System/System.Net/HttpWebRequest.cs

@@ -450,7 +450,7 @@ namespace System.Net
 					newWebHeaders.Add(headerName,webHeaders[headerName]);
 				}
 
-				webHeaders = newWebHeaders;
+				this.webHeaders = newWebHeaders;
 			}
 		}