code: public String toSecureHash() {
  		String apiKey = ApruveClient.getInstance().getApiKey();
  		String shaInput = apiKey + toValueString();
  
  		return ShaUtil.getDigest(shaInput);
  	}
docs: For use by merchants, and depends on proper initialization of
  ApruveClient. Returns the secure hash for a PaymentRequest, suitable for
  use with the property of apruve.js JavaScript library on a merchant
  checkout page. Use this to populate the value of apruve.secureHash.
  
  @return