Don't add an empty class tag if there's no class specified on the image

[moppius]

I noticed that if class is not specified in the image when generating a page using img(), HTML code gets generated like so with an empty class tag, which seems unnecessary:

<img src="http://path/to/image.jpg" class>

With this change it will instead generate:

<img src="http://path/to/image.jpg">

[justint]

Good catch, thanks Paul!