Support boolean values for additionalProperties (OpenAPITools#266)

Fixes OpenAPITools#256

Author: joschi

core/src/main/java/org/openapitools/openapidiff/core/compare/schemadiffresult/SchemaDiffResult.java

@@ -146,8 +146,8 @@ private void compareAdditionalProperties(
     Object left = leftSchema.getAdditionalProperties();
     Object right = rightSchema.getAdditionalProperties();
     if (left instanceof Schema || right instanceof Schema) {
-      Schema leftAdditionalSchema = (Schema) left;
-      Schema rightAdditionalSchema = (Schema) right;
+      Schema leftAdditionalSchema = left instanceof Schema ? (Schema) left : null;
+      Schema rightAdditionalSchema = right instanceof Schema ? (Schema) right : null;
       ChangedSchema apChangedSchema =
           new ChangedSchema()
               .setContext(context)

core/src/test/java/org/openapitools/openapidiff/core/AdditionalPropertiesTest.java

@@ -0,0 +1,16 @@
+package org.openapitools.openapidiff.core;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+import org.openapitools.openapidiff.core.model.ChangedOpenApi;
+import org.openapitools.openapidiff.core.output.ConsoleRender;
+
+class AdditionalPropertiesTest {
+  @Test
+  void booleanAdditionalPropertiesAreSupported() {
+    ChangedOpenApi diff = OpenApiCompare.fromLocations("issue-256_1.json", "issue-256_2.json");
+    ConsoleRender render = new ConsoleRender();
+    assertThat(render.render(diff)).isNotBlank();
+  }
+}