Wrong separator width in TMConstrainedHorizontalBarLayout (#563)

[stefanomondino]

fixes #563

Attached screen can be obtained with:

// Create a bar
        let bar = TMBarView.TabBar()
        
        // Customize bar properties including layout and other styling.
        bar.layout.contentInset = UIEdgeInsets(top: 0.0, left: 16.0, bottom: 4.0, right: 16.0)
        bar.fadesContentEdges = true
        bar.spacing = 16.0
        bar.layout.separatorWidth = 10
        bar.layout.showSeparators = true

[msaps]

Great job 🚀 thanks!