Correct email-alert-api data access

The type of data that gds-api-adapters returns is an OpenStruct-like
object rather than a hash.

Additionally, the expected response is actually nested inside a
`subscriber_list` top-level JSON structure.

This commit updates the model to look for methods instead of keys, and
changes the tests to use OpenStructs everywhere.
Merge pull request #50 from alphagov/use-right-response-format-in-subscription-calls

Correct email-alert-api data access