Always encapsulate command arguments in quotes

MPD accepts both quoted and unquoted command arguments but drop in replacements like Mopdiy might require quoted arguments so safer to just escape.

Only thing is that I'm not happy with the param.isEmpty() but the unit tests fail otherwise and I cannot seem to determine why, because the commandstring matches the new command names in the mockedDatabaseData.txt file. On the other hand, missing (unquoted) parameters would also cause MPD to fail the command for missing argument options, so no behaviour is changed.