Add default metadata when calling log with string level and message

[txgruppi]

The problem

When calling logger.log with two string values it does not add the values from defaultMeta.

Before

const logger = winston.createLogger({
  defaultMeta: {testing: 'it works!'},
  transports: [new Console()],
});
logger.log('info', 'did it work?'); // {"level":"info","message":"did it work?"}

After

const logger = winston.createLogger({
  defaultMeta: {testing: 'it works!'},
  transports: [new Console()],
});
logger.log('info', 'did it work?'); // {"level":"info","message":"did it work?","testing":"it works!"}

[Dinuz]

This should be push, it solves indeed the issue.

[alvinalexander]

Please merge this!