Add additional properties to Java CodegenModel (#1854)

Fixes the first part of #1729 (comment) (see #1729 (comment)). The second part was fixed by #1845 (commit d65dd76).

Author: demonfiddler

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractJavaCodegen.java

@@ -1436,4 +1436,13 @@ public void setParentVersion(final String parentVersion) {
     public void setParentOverridden(final boolean parentOverridden) {
         this.parentOverridden = parentOverridden;
     }
+
+    @Override
+    protected void addAdditionPropertiesToCodeGenModel(CodegenModel codegenModel, Schema schema) {
+        super.addAdditionPropertiesToCodeGenModel(codegenModel, schema);
+
+        // See https://github.com/OpenAPITools/openapi-generator/pull/1729#issuecomment-449937728
+        codegenModel.additionalPropertiesType = getSchemaType(ModelUtils.getAdditionalProperties(schema));
+        addImport(codegenModel, codegenModel.additionalPropertiesType);
+    }
 }