No nested default values in patch

[didier-brizet]

Only default value of first parent object is returned in patch when multiple default values are nested.

Example to reproduce the issue:

#include <iostream>
#include <nlohmann/json-schema.hpp>

using nlohmann::json;
using nlohmann::json_schema::json_validator;

int main() {
    nlohmann::json schema = R"(
    {
        "default": {},
        "properties": {
            "parent1": {
                "default": {},
                "properties": {
                    "child1": {
                        "type": "integer",
                        "default": 10
                    } 
                }
            },
            "parent2": {
                "type": "integer",
                "default": 20
            }
        }
    }
    )"_json;

    nlohmann::json_schema::json_validator validator;
    std::cout << "schema=" << schema.dump(2) << std::endl;
    validator.set_root_schema(schema);

    nlohmann::json data = R"({})"_json;
    std::cout << "data=" << data.dump(2) << std::endl;

    nlohmann::json patch = validator.validate(data);
    std::cout << "patch=" << patch.dump(2) << std::endl;
    if (!patch.empty()) {
      data = data.patch(patch);
    }

    std::cout << "data=" << data.dump(2) << std::endl;
}

schema={
  "default": {},
  "properties": {
    "parent1": {
      "default": {},
      "properties": {
        "child1": {
          "default": 10,
          "type": "integer"
        }
      }
    },
    "parent2": {
      "default": 20,
      "type": "integer"
    }
  }
}
data={}
patch=[
  {
    "op": "add",
    "path": "/parent1",
    "value": {}
  },
  {
    "op": "add",
    "path": "/parent2",
    "value": 20
  }
]
data={
  "parent1": {},
  "parent2": 20
}

Expected data is:

data={
  "parent1": {
     "child1": 10
   },
  "parent2": 20
}

[didier-brizet]

#206 (comment) solution fixes such issue whether there is no if condition or required item that depends on default values.

Default values should be appended before the validation of sub-schemas based on oneOf and allOf.
Validator must try to append default values of one sub-schema before to validate data against the sub-schema and then cancel this appending if validation failed to try a validation against to next sub-schema with different default values and so on. Currently default values of first sub-schema are appended and so on even if the validation agains a sub-schema failed. The returned patch can contain default values of one of first sub-schemas instead of default values non-first and valid sub-schema.