Corrected town/city field name

Unbelievably, 'OWNERCITY' isn't actually a valid field, which was causing orders to be rejected by ePDQ. They have 'OWNERCTY' which I believe is actually for the 'county' component of the address. As we're passing the city value, I've corrected this field to be 'OWNERTOWN'.
Merge pull request #1 from coatesap/patch-1

Corrected town/city field name.

Cheers!