fix: typeof instead of instanceof to check for object

[b-chamagne]

Hello,
I had an issue with some of the toolbar button (bold/italic) that were not working.
I think it had to do with the way I loadData(), but could not figure out what's wrong exactly.

Here's the issue from devtool (it is a style.font) :

This would make the helper.equals returns truthy instead of falsy which skip the creation of a new style (thus reuse an existing one).