Fix hiccup-like rendering of boolean attrs

[jcromartie]

When rendering elements, such as checkboxes, drop-down options, or other inputs, Hiccup will simply omit attributes that are false or nil.

The current state of html renders these attributes like checked="false" which leads to browsers interpreting it as actually being checked.

This commit updates the html function to be more compatible with Hiccup, and allows you to use Hiccup's hiccup.form namespace with Enlive.

[fdserr]

Thinking and testing needed: impact on existing apps?