[Core] Resolve Inline Models (OpenAPITools#736)

* Add InlineModelResolver.java

* fix inline schema for OAS2 body parameter

* skip warning, fix MapSchema cast error

Author: antihax

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

@@ -787,8 +787,8 @@ public List<File> generate() {
         configureOpenAPIInfo();
 
         // resolve inline models
-        //InlineModelResolver inlineModelResolver = new InlineModelResolver();
-        //inlineModelResolver.flatten(openAPI);
+        InlineModelResolver inlineModelResolver = new InlineModelResolver();
+        inlineModelResolver.flatten(openAPI);
 
         List<File> files = new ArrayList<File>();
         // models