Adds a hasOwnProperty check to ToUnicodeMap forEach method

I encountered the following console output trying to render a PDF:
```
Warning: Unsupported feature "font" pdf.worker.js:240
Warning: Error during font loading: undefined is not a function pdf.js:240
Warning: Unsupported feature "font" pdf.worker.js:240
Warning: Error during font loading: undefined is not a function pdf.js:240
```
Visually, the PDF is rendered without any text.

It turned out that the `this._map` instance member of `ToUnicodeMap` was an array and the error was coming from array prototype methods being treated as character codes.  Adding a `hasOwnProperty` check in this loop fixed the issue and my PDF looks perfect.

The PDF I'm rendering can be found in this gist:  https://gist.github.com/sfishel18/eb187c923b15017f4ee4.  Let me know if there is a better way to attach that file.