Collapse is inaccessible by keyboard if all panels are closed by default #121

There's a flaw in the logic of the collapse extension: it assumes that one of the collapsible panels is open by default (the .in class), and sets the tabindex of that panel's header to 0 so that it can be reached via keyboard. The tabindex for each of the remaining panel headers is then set to -1:

if(collpanel.hasClass('in')){
    colltab.attr({ 'aria-controls': collpanel.attr('id'), 'aria-selected':'true', 'aria-expanded':'true', 'tabindex':'0' })
    collpanel.attr({ 'role':'tabpanel', 'tabindex':'0', 'aria-labelledby':collid, 'aria-hidden':'false' })
}else{
    colltab.attr({'aria-controls' : collpanel.attr('id'), 'tabindex':'-1' })
    collpanel.attr({ 'role':'tabpanel', 'tabindex':'-1', 'aria-labelledby':collid, 'aria-hidden':'true' })
}

This doesn't work if all of the panels are collapsed by default, because then the tabindex of all the panel headers is set to -1, which makes the entire panel group inaccessible via keyboard.

To fix this, I simply set the tabindex on the headers of the collapsed panels to 0 instead of -1:

colltab.attr({'aria-controls' : collpanel.attr('id'), 'tabindex':'0' })

There don't appear to be any side effects of doing this, other than that I can now hit Tab to move between different panel headers in addition to Left/Right or Up/Down. That's fine with me, and certainly better than the entire panel group being inaccessible!