REST plugins no longer have to specify a serialization_class

Before #2419825: Make serialization_class optional all REST resource plugins required you to specify a class to which the passed in values gets converted. For example the entity resource converts the incoming data on a POST request to an entity object.

There are though some much simpler usecases like a /user/login resource, that really just needs a username and password, so converting this to an object doesn't really makes sense.

After #2419825: Make serialization_class optional 'serialization_class' is optional on the rest resource annotation. Its still encouraged to specify one, as you have more control about it.