Fix in memory logger initialization (#416)
Fix in memory logger initialization (#416)

Calling the `in_memory_logger` method will always create a new logger.
This is not useful when being called in `Appsignal.start_logger` where
we check the presence of the in memory log. Instead, check the value
instance variable `@in_memory_logger` before appending it to the new
logger.
appsignal/appsignal,appsignal/appsignal-ruby,appsignal/appsignal-ruby,matrinox/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal,matrinox/appsignal-ruby,matrinox/appsignal-ruby