Check `params` is defined

`params` is not defined in some case, for example when using ActionMailer.

```
class UserMailer < ActionMailer::Base
  def send_email_without_args
    mail(
      to:      'to@example.com',
      from:    'from@example.com',
      subject: 'Action Args!!!',
      body:    'test'
    )
  end
end

UserMailer.send_email_without_args
```
Merge pull request #5 from fukayatsu/check-params-is-defined

Check `params` is defined