Support short notation for boolean values (e.g. hx-boost instead of hx-boost="true")

According to the HTML [specification for boolean attributes](https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#boolean-attributes) it should be possible to write for example hx-boost (data-hx-boost) instead of hx-boost="true" (data-hx-boost="true").

>>The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.