Fixes & Delete AutoPay Relationship

Fixed a null reference exception when throwing an api exception where the api response contained no content.
Added support for deleting autopay/trust relationships.
Fixed 'Pending' trust status not implicitly casting or case-insensitively comparing to the string 'pending'.
Fixed some broken unit tests.

Latest API Support

• Notifications now include TransactionId (Paymark transaction reference)
• Breaking change: The OnlineEftposApiVersion.Latest enum value has been renamed OnlineEftposApiVersion.None and is the default. The latest Paymark API no longer uses versions in the URLs, specifying OnlineEftposApiVersion.None will use the new versionless endpoints. Specifying OnlineEftposApiVersion.V1P1 will still use the old v1 urls, but these will be retired shortly.