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
Allow requests which accept any mime-type response
Merge pull request #51 from alphagov/wildcard-accepts

Allow requests which accept any mime-type response
Subscribe to "topics" instead of "topic".

This is in line with the tags sent from both publishers
to the content store, and the content store docs
themselves.
Merge pull request #52 from alphagov/subscribe_to_topics_instead_of_topic

Subscribe to "topics" instead of "topic".