Added 'default' parameters in {JmsMetadata,Validator}Parser, and FOSRestHandler

Related to https://github.com/nelmio/NelmioApiDocBundle/pull/408

As far as I am aware, `jms-serializer` nor `symfony-validator` doesn't have a concept of default values, nor is it possible to retrieve the default value of a property in a class via reflection. So this PR simply assigns `null` to `default`.

I have added a `default` property to parameters produced by `FOSRestHandler` whenever appropriate.