title: UITableViewHeaderFooterView contentView is not setting to transparent
body: <p>I've a ViewController with a background image which as a TableVC as its subview. 
  I want the table view to be transparent and have accomplished that, also the tableviewcell's are transparent. But I'm facing a difficulty in getting the tableviewheaderfooterview to be transparent, as when i set it's contentview backgroundcolor to clearColor - it shows a gray color.  </p>
  
  <p>I'm using Xcode 7.3.1</p>
  
  <pre><code>private let collapsibleTableView : UITableView = {
     let tv = UITableView()
      tv.translatesAutoresizingMaskIntoConstraints = false
      tv.backgroundColor = .clearColor()
      tv.backgroundView = nil
      tv.separatorStyle = .None
      return tv
  }()
  </code></pre>
  
  <p>Also, I've subclassed UITableViewHeaderFooterView to my custom class and where I also tried setting the contentView background color to clearColor </p>
  
  <pre><code>override func layoutSubviews() {
          super.layoutSubviews()
          contentView.backgroundColor = .clearColor()
  }
  </code></pre>
  
  <p>Also, I've tried to set it again in my VC file </p>
  
  <pre><code>func tableView(tableView: UITableView, viewForHeaderInSection section: Int) -&gt; UIView? {
          let header = tableView.dequeueReusableHeaderFooterViewWithIdentifier("header") as? CustomTableViewHeader ?? CustomTableViewHeader(reuseIdentifier: "header")
          header.contentView.backgroundColor = .clearColor()
          header.titleLabel.text = "custom-text"
          header.delegate = self
          return header
      }
  </code></pre>
  
  <p>Also, I've set the tableviewcell's background to transparent too </p>
  
  <pre><code>func tableView(tableView: UITableView, willDisplayCell cell: UITableViewCell, forRowAtIndexPath indexPath: NSIndexPath) {
  
              cell.backgroundColor = .clearColor()
              cell.contentView.backgroundColor = .clearColor()
  }
  </code></pre>
  
  <p>But no matter what I do, the background of the headerview will not be transparent. </p>
  
  <p><a href="https://i.stack.imgur.com/UImVu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UImVu.png" alt="enter image description here"></a></p>
  
  <p>EDIT: I also found this <a href="https://stackoverflow.com/questions/33118178/custom-uitableview-header-set-background-colour-fail">post</a> about the same problem here on SO, but implementing the proposed solution, isn't working</p>
  
  <pre><code> header.contentView.layer.backgroundColor = UIColor.clearColor().CGColor
  </code></pre>