Undeclared variables throw ReferenceError in Webpack

[volkanunsal]

I've noticed this error more than a few times when I tried to use the latest version, and once or twice with version 0.9.0. There are a number of libraries that jsPDF includes that try to assign to a variable without declaring it first. These would be handled gracefully in the browser, but Webpack is throwing a ReferenceError.

[Flamenco]

see #529 for fix