explicitly check if devicePixelRatio cookie hasn't been set

hi, we use your gem on two sites and we started to have an issue where the cookies that google analytics adds to our pages were causing the retina_image_tag javascript block not to execute (as document.cookie was not blank). 

i've changed the test to explicitly check for the devicePixelRatio entry rather than just checking for blankness of the document.cookie variable (i also do the redirect based on the window.devicePixelRatio, rather than the resulting document.cookie string, for the same reason)

this is my first pull request, please be gentle :)