fix(clients): InvalidOperationException in C# Fill() due to unsafe dictionary enumeration (#4820)

Co-authored-by: Renee Mundie <reneemundie@PEO-M3P-RM.local>
Co-authored-by: Morgan Leroi <morgan.leroi@gmail.com>

Author: 3 people

clients/algoliasearch-client-csharp/algoliasearch/Http/HttpRequestHeadersExtensions.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Linq;
 using System.Net.Http.Headers;
 using Algolia.Search.Utils;
 
@@ -14,7 +15,7 @@ internal static class HttpRequestHeadersExtensions
   /// <returns></returns>
   internal static void Fill(this HttpRequestHeaders headers, IDictionary<string, string> dictionary)
   {
-    foreach (var header in dictionary)
+    foreach (var header in dictionary.ToList())
     {
       headers.TryAddWithoutValidation(header.Key, header.Value);
     }