beamer does not honour \setmainfont, how to fix? [duplicate]

Question

I have a local set of .ttf files which I'd like to use in my presentation.

When I create an article document the font gets selected properly:

    \documentclass[12pt,a4paper]{article}

    \usepackage{fontspec}
    \setmainfont [ Path = fonts/open-sans/, 
        Extension      = .ttf,
        UprightFont = *-Regular,
        BoldFont = *-Semibold,
        BoldItalicFont = *-SemiboldItalic,
        ItalicFont = *-Italic
    ] {OpenSans}

    \usepackage{blindtext}

    \begin{document}
            1234567890 
            \blindtext
    \end{document}

But when I change this to a beamer presentation I end up with a different font:

    \documentclass{beamer}

    \usepackage{fontspec}
    \setmainfont [ Path = fonts/open-sans/, 
        Extension      = .ttf,
        UprightFont = *-Regular,
        BoldFont = *-Semibold,
        BoldItalicFont = *-SemiboldItalic,
        ItalicFont = *-Italic
    ] {OpenSans}

    \usepackage{blindtext}

    \begin{document}
        \begin{frame}
            1234567890 
            \blindtext
        \end{frame}
    \end{document}

Here you see the difference:

The lower text is the right font in article mode, the upper one the beamer standard.

I even tried to use \setbeamerfont but this didn't change anything.

Can somebody please point me how to convince the beamer class to use the provided font files?

This is my environment:

$ lualatex --version
This is LuaTeX, Version beta-0.80.0 (TeX Live 2015/Debian) (rev 5238)

$lualatex testfonds-article.tex | grep "Document Class: beamer"
Document Class: beamer 2015/01/05 3.36 A class for typesetting presentations (rc