Worldpay: XML encoding must be ISO-8859-1

This allows for proper parsing of non-Latin characters. Confirmed in
practice and by Worldpay Engineers on their support line.

Closes #578.
Worldpay: Add card codes for supported card types

Fixes a bug where supported card types had no card-code associated with
them. Card types added: JCB, Maestro, Laser.

Closes #579.
Ogone: Allow specifying action as an option

Default action SAL was hard-coded but now it's possible to pass it as an
option to be able to use SAS.

Closes #580.
PayPal Express gateway: Add BuyerEmailOptInEnable

* Added option to enable buyer email opt-in
* Added BuyerEmailOptInEnable element
* Updated tests to include use of this element

Closes #584.
Individual gateways work with :test => false

If Base.test? was true but a developer attempted to override it by
specifying :test => false in an individual gateway's options, it
continued to operate in test mode due to interaction of the false value
with the || operator being used. This instead always uses the gateway's
:test option if it is set in the options hash.

Reported by @DiegoSalazar. Fixes #586.
Add Paymill gateway

https://www.paymill.com

Closes #587.
Merge remote branch 'upstream/master' into finansbank_gateway