Fixed checkConditions when an expected value is supplied.

According to the ruby implementation of fake_dynamo, when a value
is supplied the exist flag should be checked for both undefined
and true (since it is not required if value is supplied):

  if exist.nil? or exist
    raise ConditionalCheckFailedException unless (old_item and old_item[name] == expected_attr)

In addition, the ruby version overrides the == operator so that
the comparison test succeeds. The node version should use the
equals method instead of !==.
Flipped the equality test in case oldItem.attributes[name] is undefined.