**Why**:
When looking up a class with `Object.const_defined?`, `false` will be
returned the first time it is called, even when the class exists in the
Rails app. I think this might be due to the way Rails loads classes.
**How**:
Use `ActiveSupport::Inflector#constantize`, which returns the class all
the time when the class exists, and throws a `NameError` when it
doesn't.
The only way I was able to properly test this was to create the
`UserOtpSender` class as a real file in the test Rails app, and create
a Devise Admin user to test the scenario where `AdminOtpSender` does
not exist.
I verified that with the old code, `reset_otp_state` was not being
called when it should be, and that the new code makes the tests pass.
Moncef Belyamani