conflicts
DRY-up setting of the content-available field

A NewsstandNotification just merges in the value and lets the base
Notification do the work.
Failing test case for issue with content-available in using Grocer::Server.
Clean bad payload keys when reading Notifications

NotificationReader will sanitize payload keys, replaceing '-' with '_'
before passing the payload on to the Notification object.

Closes #59
tiny patch up based on @stevenharman's review request
valid? should return truthy or falsey values
Convention dictates that methods ending in a '?' return a truthy or falsey value.
Previously 'valid?' would return nil (which evaluates to false) if valid, and raise and exception if invalid.
remove redundant true
push truthiness into validate_payload for improved readability