Fix EZP-21589: Boolean are serialized as string in JSON resp. of the REST API

# Description

The boolean values in the JSON REST API responses are returned as string ("false" or "true") instead of *true* boolean. As a result, you need to do string comparison client side to test a boolean...

This patch adds a new methods to the generator so that the boolean values are correctly serialized depending on the output type.

# Tests

unit tests + manual tests on `content/types/XX` and `content/objects/XX`