Prepare a record for queuing. The object returned by this method is
enqueued.
The base implementation formats the record to merge the message and
arguments, and removes unpickleable items from the record in-place.
Specifically, it overwrites the record's `msg` and
`message` attributes with the merged message (obtained by
calling the handler's `format` method), and sets the `args`,
`exc_info` and `exc_text` attributes to None.
You might want to override this method if you want to convert
the record to a dict or JSON string, or send a modified copy
of the record while leaving the original intact.