fixed to work with netstandard2.0

Author: Pjort

Supabase/Client.cs

@@ -274,9 +274,9 @@ internal Dictionary<string, string> GetAuthHeaders()
 
             // Add custom headers from options
             // This will overwrite any existing headers with the same key, not sure if that's the desired behavior
-            foreach (var (key, value) in _options.Headers)
+            foreach (var kvp in _options.Headers)
             {
-                headers[key] = value;
+                headers[kvp.Key] = kvp.Value;
             }
 
             return headers;