Add Postmark::ApiClient#serialize method.
Always create a new API client instance in Mail::Postmark handler.
Allow changing api key on existing HttpClient instances.
Add full_response attribute to DeliveryError.
Save full response body when raising DeliveryErrors.
Return self from Mail::Postmark delivery handler.
Add delivered? predicate and delivered and postmark_response attr_accessors to Postmark::SharedMessageExtensions module.
Make Postmark::ApiClient always save Postmark API's response to the message (even if delivery failed).
Update integration specs accordingly to the recent changes.