Skip implicit layouts for nested templates.

That way the following will produce valid HTML:

@@ layout
!!!
= yield

@@ content
%html
  %head= haml :head
  %body= haml :body

That way using render methods for partials is a lot easier.
Tests included.

lib/sinatra/base.rb

@@ -371,14 +371,18 @@ module Sinatra
       options[:outvar] ||= '@_out_buf'
 
       # extract generic options
-      locals = options.delete(:locals) || locals || {}
-      views = options.delete(:views) || settings.views || "./views"
-      layout = options.delete(:layout)
-      layout = :layout if layout.nil? || layout == true
+      locals          = options.delete(:locals) || locals         || {}
+      views           = options.delete(:views)  || settings.views || "./views"
+      @default_layout = :layout if @default_layout.nil?
+      layout          = options.delete(:layout)
+      layout          = @default_layout if layout.nil? or layout == true
 
       # compile and render template
-      template = compile_template(engine, data, options, views)
-      output = template.render(self, locals, &block)
+      layout_was      = @default_layout
+      @default_layout = false if layout
+      template        = compile_template(engine, data, options, views)
+      output          = template.render(self, locals, &block)
+      @default_layout = layout_was
 
       # render layout
       if layout

test/templates_test.rb

@@ -78,6 +78,27 @@ class TemplatesTest < Test::Unit::TestCase
     assert_equal "Layout 3!\nHello World!\n", body
   end
 
+  it 'avoids wrapping layouts around nested templates' do
+    render_app { render :str, :nested, :layout => :layout2 }
+    assert ok?
+    assert_equal "<h1>String Layout!</h1>\n<content><h1>Hello From String</h1></content>", body
+  end
+
+  it 'allows explicitly wrapping layouts around nested templates' do
+    render_app { render :str, :explicitly_nested, :layout => :layout2 }
+    assert ok?
+    assert_equal "<h1>String Layout!</h1>\n<content><h1>String Layout!</h1>\n<h1>Hello From String</h1></content>", body
+  end
+
+  it 'two independent render calls do not disable layouts' do
+    render_app do
+      render :str, :explicitly_nested, :layout => :layout2
+      render :str, :nested, :layout => :layout2
+    end
+    assert ok?
+    assert_equal "<h1>String Layout!</h1>\n<content><h1>Hello From String</h1></content>", body
+  end
+
   it 'loads templates from source file' do
     mock_app { enable :inline_templates }
     assert_equal "this is foo\n\n", @app.templates[:foo][0]

test/views/explicitly_nested.str

@@ -0,0 +1 @@
+<content>#{render :str, :hello, :layout => :layout2}</content>

test/views/hello.str

@@ -0,0 +1 @@
+<h1>Hello From String</h1>

test/views/layout2.str

@@ -0,0 +1,2 @@
+<h1>String Layout!</h1>
+#{yield}

test/views/nested.str

@@ -0,0 +1 @@
+<content>#{render :str, :hello}</content>