Test for correct handling of "download" attribute

These tests currently fail as there is no special treatment for this kind of
attribute.

Related: GH-1337
Support minimizable, non-boolean attributes

Fixes GH-1337
Rename CAN_BE_MINIMIZED to HAS_BOOLEANISH_VALUE
Combine valid value checks
Test more falsey values
Rename "booleanish" to "overloaded boolean"
Merge pull request #1340 from matthewwithanm/download-attribute

Support download attribute properly