Fix ASP0019 warning (#7107)

Co-authored-by: Michael Staib <[email protected]>

Author: glen-84

src/HotChocolate/AzureFunctions/test/HotChocolate.AzureFunctions.Tests/Helpers/TestHttpContextHelper.cs

@@ -47,9 +47,7 @@ public static HttpContext NewBcpHttpContext()
         request.QueryString = new QueryString(uri.Query);
 
         // Ensure we accept Text/Html for BCP requests...
-        httpContext.Request.Headers.Add(
-            HeaderNames.Accept,
-            TestConstants.DefaultBcpContentType);
+        httpContext.Request.Headers[HeaderNames.Accept] = TestConstants.DefaultBcpContentType;
 
         httpContext.Response.Body = new MemoryStream();