Fix hiccup-like rendering of boolean attrs

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.