Use the default layout when calling :layout => true

This helps to write things like:

  haml :template, :layout => !request.xhr?

lib/sinatra/base.rb

@@ -210,6 +210,7 @@ module Sinatra
 
     def lookup_layout(engine, options)
       return if options[:layout] == false
+      options.delete(:layout) if options[:layout] == true
       template = options[:layout] || :layout
       data = lookup_template(engine, template, options)
       [template, data]

test/templates_test.rb

@@ -52,6 +52,14 @@ describe 'Templating' do
     end
   end
 
+  it 'uses the default layout template if not really overriden' do
+    with_default_layout do
+      render_app { render :test, :hello, :layout => true }
+      should.be.ok
+      body.should.equal "Layout!\nHello World!\n"
+    end
+  end
+
   it 'uses the layout template specified' do
     render_app { render :test, :hello, :layout => :layout2 }
     should.be.ok