allow nil in class and remove it

[jbswetnam]

Great library! Just stumbled on it today, and I like the approach. One additional feature that I would like to have is the ability to set the class of an element conditionally without unwanted spaces or an empty class attribute.

This update allows that:

(->html (div :class (if false "won't-appear")))
; "<div></div>"
; old version: "<div class=\"\"></div>"

(->html (div :class ["this-appears" (if false "won't-appear")]))
; "<div class=\"this-appears\"></div>"
; old version: "<div class=\" this-appears\"></div>"

(->html (div :class [(if false "won't-appear")]))
; "<div></div>"
; old version: "<div class=\"\"></div>"

I did not change the cljs source because I don't know how to run the tests for it (still new at cljs). I'd be happy to update it if you can tell me how to run the tests!