message: fix: wrong warnings for $ref not single
  closes
diff: @@ -99,6 +99,7 @@ class SchemaWalker {
  export class AllOfMerger {
  static merge(into, schemas) {
  into['x-derived-from'] = [];
  + let hadDiscriminator = !!into.discriminator;
  for (let i=0; i < schemas.length; i++) {
  let subSchema = schemas[i];
  into['x-derived-from'].push(subSchema._pointer);
  @@ -115,7 +116,7 @@ export class AllOfMerger {
  defaults(into, subSchema);
  subSchema._pointer = tmpPtr;
  }
  - into.discriminator = null;
  + if (!hadDiscriminator) into.discriminator = null;
  into.allOf = null;
  }
  @@ -214,7 +215,8 @@ class SchemaDereferencer {
  // if resolved schema doesn't have title use name from ref
  resolved.title = resolved.title || JsonPointer.baseName($ref);
  - let keysCount = Object.keys(schema).length;
  + let keysCount = Object.keys(schema).filter(key => !key.startsWith('x-redoc')).length;
  +
  if ( keysCount > 2 || (keysCount === 2 && !schema.description) ) {
  WarningsService.warn(`Other properties are defined at the same level as $ref at "#${pointer}". ` +
  'They are IGNORED according to the JsonSchema spec');
  