Fix a bug with dfonts not loading correctly

See foliojs/pdfkit#245 for information on this

src/printer.js

@@ -410,7 +410,7 @@ FontProvider.prototype.provideFont = function(familyName, bold, italics) {
  if (cached) return cached;
 
  var fontCache = (this.cache[familyName] = this.cache[familyName] || {});
- fontCache[type] = this.pdfDoc.font(this.fonts[familyName][type])._font;
+ fontCache[type] = this.pdfDoc.font(this.fonts[familyName][type], familyName)._font;
  return fontCache[type];
 };