/// <summary>
/// Gets the Gravatar URL.
/// </summary>
/// <param name="email">The email of user.</param>
/// <param name="secure">Use HTTPS?</param>
/// <param name="size">The Gravatar size.</param>
/// <param name="rating">The Gravatar rating.</param>
/// <returns>A gravatar URL.</returns>
public static string GetURL(string email, int size = 150,
string rating = "x") => $"{HttpsUrl}{MD5Core.GetMD5String(email)}&s={size.ToString(CultureInfo.InvariantCulture)}&r={rating}";