Make the CSP header be bytes not unicode
    
    Header names must be byte string not unicode string, so we need
    to make sure our CSP header complies with that.
    
    Otherwise we run into:
      Apache returning a 500 error:
        TypeError: expected byte string object for header name,
        value of type unicode found
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>