[BUGFIX] PayDirekt: Add purpose parameter

[thommyhh]

No description provided.

[judgej]

Thank you. So I understand correctly (and after trying it out on the sandbox), a capture of a PayDirekt authorisation requires the purpose field, which is derived from the description parameter in Omnipay. Without that field being present - whether empty or not (empty is permitted) the gateway will fail to cpature with error "Pflichtfeld nicht angegeben" ("Mandatory field not specified").

I think there was some ambiguity in the docs in this area at the time this was first written, so I'll revisit it now and see if there are any special requirements here for capture of other payment types. Also some tests covering this area would be good.