[FIX] Convertion from DS_HASH_TABLE keys to JSON

This is useful when the type of the key is something else than a STRING
and we have a JSON converter for this type.

library/gobo/converters/json_ds_hash_table_converter.e

@@ -64,7 +64,11 @@ feature -- Conversion
             until
                 c.after
             loop
+            	if attached {JSON_STRING} json.value (c.key) as l_key then
+            		js := l_key
+            	else
                 	create js.make_json (c.key.out)
+            	end
                 jv := json.value (c.item)
                 if jv /= Void then
                     Result.put (jv, js)