AWS::Record::HashModel replace reference to id attribute with call to ha...

...sh_key

When a developer defines a hash key attribute not named "id", retrieving an existing item fails at dynamo_db_item:152 with: AWS::Record::UndefinedAttributeError: id.  Replacing the reference to id attribute with call to instance method hash_key (line 31) fixes this error as the hash_key method utilizes the attribute that was defined with option :hash_key.