logPossibleMemoryLeak throws an error when event name is a Symbol

[doronguttman]

When eventName is a Symbol AND maxListeners threshold is exceeded AND verboseMemoryLeak is true, the following error will be thrown:

TypeError: Cannot convert a Symbol value to a string
      at EventEmitter.logPossibleMemoryLeak (node_modules/eventemitter2/lib/eventemitter2.js:60:35)
      at EventEmitter._on (node_modules/eventemitter2/lib/eventemitter2.js:1267:31)
      at EventEmitter.on (node_modules/eventemitter2/lib/eventemitter2.js:1178:17)

This is caused by the following line:

      errorMsg += ' Event name: ' + eventName + '.';

as a Symbol cannot be implicitly converted to a string. see here.