Add layout_engine option to #render.

This will render an erb file with a haml layout:

    get('/') { erb :index, :layout_engine => :haml }

Whereas this will cause all markdown templates to be rendered with an erb
layout (unless, of course, `:layout_engine` is set explicitly):

    set :markdown, :layout_engine => :erb

Fixes #110.

lib/sinatra/base.rb

@@ -468,7 +468,8 @@ module Sinatra
       layout          = options.delete(:layout)
       eat_errors      = layout.nil?
       layout          = @default_layout if layout.nil? or layout == true
-      content_type    = options.delete(:content_type) || options.delete(:default_content_type)
+      content_type    = options.delete(:content_type)  || options.delete(:default_content_type)
+      layout_engine   = options.delete(:layout_engine) || engine
 
       # compile and render template
       layout_was      = @default_layout
@@ -480,7 +481,7 @@ module Sinatra
       # render layout
       if layout
         options = options.merge(:views => views, :layout => false, :eat_errors => eat_errors)
-        catch(:layout_missing) { output = render(engine, layout, options, locals) { output }}
+        catch(:layout_missing) { output = render(layout_engine, layout, options, locals) { output }}
       end
 
       output.extend(ContentTyped).content_type = content_type if content_type

test/templates_test.rb

@@ -208,6 +208,22 @@ class TemplatesTest < Test::Unit::TestCase
     assert ok?
     assert_equal 'template in subclass', body
   end
+
+  it "is possible to use a different engine for the layout than for the template itself explicitely" do
+    render_app do
+      settings.template(:layout) { 'Hello <%= yield %>!' }
+      render :str, "<%= 'World' %>", :layout_engine => :erb
+    end
+    assert_equal "Hello <%= 'World' %>!", body
+  end
+
+  it "is possible to use a different engine for the layout than for the template itself globally" do
+    render_app :str => { :layout_engine => :erb } do
+      settings.template(:layout) { 'Hello <%= yield %>!' }
+      render :str, "<%= 'World' %>"
+    end
+    assert_equal "Hello <%= 'World' %>!", body
+  end
 end
 
 # __END__ : this is not the real end of the script.