Allow value-less dash-cased props

Currently you can't pass down dash-cased properties without a value.

See error case here in REPL.

// current
{ 'data-foo' }

// w/ update
{ 'data-foo': true }

Changes

• Add true to output if contains dashed and non-value
• Refactor if/else into named functions
• data-attribute -> any dash-cased prop
• Add test