Fix an error with string replace() method on attributes caused by #198

lib/svgo/js2svg.js

@@ -259,7 +259,7 @@ JS2SVG.prototype.createAttrs = function(elem) {
         attrs +=    ' ' +
                     attr.name +
                     this.config.attrStart +
-                    attr.value.replace(/"/g, '"') +
+                    String(attr.value).replace(/"/g, '"') +
                     this.config.attrEnd;
 
     }, this);