src: make Object::GetPropertyNames() const

Fixes: #380
PR-URL: #415
Reviewed-By: Gabriel Schulhof <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>

Author: DaAitch

napi-inl.h

@@ -1078,7 +1078,7 @@ inline bool Object::Delete(uint32_t index) {
   return result;
 }
 
-inline Array Object::GetPropertyNames() {
+inline Array Object::GetPropertyNames() const {
   napi_value result;
   napi_status status = napi_get_property_names(_env, _value, &result);
   NAPI_THROW_IF_FAILED(_env, status, Array());

napi.h

@@ -601,7 +601,7 @@ namespace Napi {
       uint32_t index ///< Property / element index
     );
 
-    Array GetPropertyNames(); ///< Get all property names
+    Array GetPropertyNames() const; ///< Get all property names
 
     /// Defines a property on the object.
     void DefineProperty(