Fix issue with keys ending to dot '.'

- match_to_key method adds '*' at the end of the key, which makes
  valid_key? to return false, since '*' is not in the list of valid
  characters defined in VALID_KEY_RE
- because the key is invalid, it is ignored
Make @ a valid character in keys

- In Rails views, there might be instance variables, e.g.
  t('payment.status.#{@payment_status}')
Merge branch 'allow-instance-variables'