Allow required to overwrite required_by_default

[geryogam]

In the README.md file, the required_by_default option is described as:

If true, all schemas that don't explicitly set the required property will be required.

But schemas that explicitly set the required property do not overwrite the required_by_default option as advertised, so they are always required (even with required = false using JSON Schema 3 syntax or required = [] using JSON Schema 4 syntax).

The issue can be demonstrated with this code snippet which does not pass validation:

var element = document.querySelector("#editor");
var options = {
  "required_by_default": true,
  "schema": {
    "type": "object",
    "properties": {
      "a": {"type": "string"},
      "b": {"type": "string"}
    },
    "required": ["a"]  // should overwrite `required_by_default` but does not
  }
};
var editor = new JSONEditor(element, options);
var errors = editor.validate({"a": "foobar"});

if (errors.length) {
  window.console.log(errors);
} else {
  window.console.log("OK");
}

[schmunk42]

https://github.com/jdorn/json-editor#deprecation-notice

[geryogam]

@schmunk42 Done: json-editor/json-editor#42