Whitelist only `for` and `class` attributes on BooleanInput label.

I can't find a decent reason to be merging with `input_html_options` and fussing around with it like it was.

Closes #995
Merge pull request #1016 from justinfrench/whitelist-boolean-label-html-options

Whitelist only `for` and `class` attributes on BooleanInput label.