code: public Gravatar setStandardDefaultImage(DefaultImage standardDefaultImage) {
          assert standardDefaultImage != null;
  
          this.standardDefaultImage = standardDefaultImage;
          customDefaultImage = null;
          return this;
      }
docs: Set the default image which will be retrieved when there is no avatar
  for the given email, when the avatar can't be shown due to the rating
  or when you enforce the default avatar.
  
  @param standardDefaultImage One of multiple default avatar choices.
  @return Fluent interface