fix dynamodb-toolbox#42 by adding undefined check to default values

Author: jeremydaly

lib/parseMapping.js

@@ -80,7 +80,7 @@ module.exports = (field,config,track) => {
     && typeof config.coerce === 'undefined') config.coerce = true
 
   // Set defaults
-  if (config.default) track.defaults[field] = config.default
+  if (config.default !== undefined) track.defaults[field] = config.default
 
   // Track required settings
   if (config.required === true) track.required[field] = false