Auto-flush in 'Detailed' log format

Even with wai-extra-2.0.3.1, the logging in "Detailed" format (eg yesod devel) is not flushed promptly. Not even as promptly as Apache or CustomOutputFormat!  This is about the simplest possible fix to improve it.

The criticism of this fix might be that it is too eager to flush, but I imagine Detailed logging is pretty exclusively used in development, where immediate flushing is desirable.  As far as I can see, anything else would require a configuration parameter to control flushing options.