Give collapsed bs-panel "collapsed" class initially

[AGresvig]

When using the bs-panel as a collapsible panel that is closed initially, like this:

{{#bs-panel heading="My Heading" collapsible=true open=false}}

..then the "collapsed" class is not added to the panel initially.

Suggestion:
The following line in bs-panel.hbs

<a class="accordion-toggle" data-toggle="collapse" data-parent="#accordion" {{bind-attr href=collapsibleBodyLink}}>

could look like this to solve the problem:

<a {{bind-attr class=":accordion-toggle open::collapsed"}} data-toggle="collapse" data-parent="#accordion" {{bind-attr href=collapsibleBodyLink}}>

Perhaps you'd prefer this to be solved through JS, but say the word and I'll submit a PR with this fix.

[AGresvig]

For anyone interested, I'd like the class to be added initially because I use styling to apply an up/down icon to the panel header, depending on the state.

SASS Example:

//Add down/up arrows to collapsible panel headers
.ember-view .accordion-toggle {
  &:before {
    content: "\e113";
    font-family: "Glyphicons Halflings";
    padding-right: 10px;
    display: inline-block;
    vertical-align: middle;
    cursor: pointer;
  }
  &.collapsed:before {
    content: "\e114";
  }
}