Fix model inheritance.

Author: warnyul

modules/swagger-codegen/src/main/resources/objc/model-header.mustache

@@ -16,7 +16,7 @@
 @protocol {{classname}}
 @end
 
-@interface {{classname}} : {{classPrefix}}Object
+@interface {{classname}} : {{#parent}}{{{parent}}}{{/parent}}{{^parent}}{{classPrefix}}Object{{/parent}}
 
 {{#vars}}
 {{#description}}/* {{{description}}} {{^required}}[optional]{{/required}}