Adding capture method for Data Cash gateway requests.

Hi,

A client of ours has their data cash configured to allow both ecommerce and telephone ordering to be used. 

When used in this way Data Cash are unable to automatically detect the default capture method, which results in the following error... https://datacash.custhelp.com/app/answers/detail/a_id/318/~/return-code-190---no-capture-method-specified

You can see here... https://datacash.custhelp.com/app/answers/detail/a_id/818 the details of how Data Cash go about detecting the capture method when multiple capture methods are enabled in the Data Cash account settings.

In this instance (I am capturing for an ecommerce store, and wish the capture method to identify that), for Data Cash to accept the request the capture type needs to be explicitly set to 'ecomm'. This is what I have added to each request being made, however I believe this may cause errors for anyone who is using active merchant to process telephone orders.

I think the PR should therefore be edited to allow for an optional capture method setting to be defined when initialising the gateway, however I'm unsure how optional settings such as this are implemented in activemerchant. If someone could give me a hint ill be happy to add another commit to make this setting optional.