Without binmode strings with incompatible encoding can't be written in
the file. This is very common in applications that log user provided
parameters.
We need to allow changing the binnary mode because right now it is impossible to use
the built-in log rotation feature when you provide a File object to the
LogDevice, and if you provide a filename you can't have binmode.
https://github.com/ruby/logger/commit/9114b3ac7e