Recognise cases when a zero attribute value is legitimate

When tag attributes are parsed, any whose value isn't equivalent to "true" is discarded. However, as the value attribute can legitimately be zero or and empty string, it's white-listed.

However, it's not the only attribute that can legitimately have a zero value. Min and max for a number type input field  may be required to impose a zero bound on the field's input. So these have been added to the white-list, along with some associated tests.