Mono files aborted with mid/side setting

[PhilButterworthSP]

Hi Beschulz

This is working great apart from an issue I have with mono files when using the "--channels mid" or "side" options. The readme states:

When the input is mono, the application falls back to outputting the left channel and prints a warning to stderr.

I get the first error but it's followed by a second:

Warning: your trying to generate output for channel 'mid', but the input has only one channel. removing requested channel. Warning: there are no channels left to process, aborting.

The resulting JSON contains an empty array: "mid":[]. Using the "--channels left" option works as expected.