add the accept header to the request generator method in Ruby

[baywet]

The request generator method needs to also include the accept header to disambiguate between content types for APIs that do support multiple content types.
More details #1607

[baywet]

Examples
https://github.com/microsoft/kiota-samples/blob/d3f5a537a8bff85bda1f7fe42ba848e99f3646f4/msgraph-mail/dotnet/Users/Item/Messages/MessagesRequestBuilder.cs#L63

kiota/src/Kiota.Builder/Writers/CSharp/CodeMethodWriter.cs

Line 268 in 889476c

writer.WriteLine($"{RequestInfoVarName}.Headers.Add(\"Accept\", \"{string.Join(", ", codeElement.AcceptedResponseTypes)}\");");