Ensure additional properties are on by default

Anyone embedding this library AND extending the DefaultGenerationConfig
will have additional properties enabled again with this change.

Closes joelittlejohn#398.

jsonschema2pojo-core/src/main/java/org/jsonschema2pojo/DefaultGenerationConfig.java

@@ -227,6 +227,6 @@ public boolean isConstructorsRequiredPropertiesOnly() {
 
     @Override
     public boolean isIncludeAdditionalProperties() {
-        return false;
+        return true;
     }
 }