SagePay: Truncate description field

If a description longer than 100 characters is specified, SagePay fails
the transaction with this error message:

3082 : The Description value is too long.

Now we truncate the description to match SagePay's constraint.  This
simplifies the code of those using Active Merchant since they won't need
to conditionally account for SagePay's description contraint.

Closes #1017.