Test for explitily setting layout true

See #727

If the default options for a template include :layout => false, it
should be possible to override this in the explicit options to the
rendering method.

e.g.

    set :slim, :layout => false

    get '/' do
      slim :index, :layout => true # should render with layout
    end
Allow overriding :layout => false with true

Fixes #727

The :layout option for templates controls both which template should be
used as the layout and whether a layout should be used at all.

If the default options for an engine include :layout => false, then
passing in :layout => true in the call to the render method shouldn't
use false as the value of :layout, it should fall back to
@default_layout (the "default default").
Merge pull request #728 from mattwildig/false-layouts

Allow overriding global :layout => false setting with true on per-method basis