Error: Cannot parse (PLAIN) into a raw HTML command: charsetName

[tresf]

As part of #978, some major refactors to the handling of encoding caused a regression.

To reproduce:

var data = [{
    type: "raw",
    format: "html",
    flavor: "plain",
    data: "<h1>Hello</h1>",
    options: { language: "ZPL" }
}];

var config = qz.configs.create("Zebra");

qz.print(config, data).catch(e => console.error(e));

Workaround:

Specify encoding:

- var config = qz.configs.create("Zebra");
+ var config = qz.configs.create("Zebra", { encoding: "UTF8" );