Twig extension: Add functionality to getResizedImage to not include width and height attributes

[joeholdcroft]

When you call getResizedImage() within a Twig file, the image tag is rendered with width and height attributes.

This is almost always handy to avoid janky page loading, but in some cases (such as when you're doing something responsive) you don't want this as it's impossible(?) to override with CSS.

You can pass an associative array of attributes as the 4th parameter for this function, but passing width or height would result in two width or height parameters.

Update the code to watch for these attributes, and if they are defined with a value of null, perhaps, don't render the attributes.

[joeholdcroft]

This first came up on the Mothership Demo when @aislingbrock was making the carousel responsive.