externals value as array fails

Hi, 
Thanks for the wonderful work you guys are doing. 
I've encountered a problem with externals configuration
The documentation [here](https://webpack.js.org/configuration/externals/#array) says the value of a property under the externals object can be an array - the path to the resource.
However, webpack fails if an array is used. 