FireFox does not like .attr('src', null)

[dlmiles]

https://github.com/jetmartin/responsive-lazy-loader/blob/master/js/jquery.responsivelazyloader.js#L283

This is not liked by FireFox. It appears to try to load the referencing URL the was defined in. This is the same as if .attr('src', '') which also does the same. I have to comment out this line to stop it hitting the target server an extra time once for each use of <img data-src="something"/>