Checkout form fields sending longer values than Sagepay Server Protocol and Integration Guidelines allow.

How to reproduce

1. Populate your cart with some products
2. Proceed through checkout until the Billing and Delivery information
3. Enter values longer than 20 charachers in any of the following: billing_first_name / billing_last_name / billing_phone / delivery_first_name / delivery_last_name / delivery_phone
4. Proceed through to order review and submit

What should happen

You should be sent through to sagepays payment page where you select your card type and then proceed through to entering details.

What actually happens

You are redirected back to the /cart/checkout page with no errors, trying the same details again and the customer enters an endless loop.

The file attached is sagepays server integration guidelines. Page 42 and 43 document the #maxlengths accepted.

DPR of $form[billing_first_name] showing maxlength = 30;
[billing_first_name] => Array
(
[#type] => textfield
[#title] => First name
[#description] =>
[#size] => 32
[#maxlength] => 32
[#required] => 1
[#default_value] =>
[#summary] => First name is not set.
)