Fix rails#5440 - multiple render_to_string breaks partials formats

This fixes situation where rendering template to string
sets `rendered_format` to the format rendered there.
This is ok to have consistent formats rendered in partials,
but it breaks on next renders if format is explicitly set
or on last render where default format does not necessarily
need to be the format of first rendered template.

actionpack/lib/abstract_controller/rendering.rb

@@ -106,6 +106,7 @@ def render_to_body(options = {})
     # Find and renders a template based on the options given.
     # :api: private
     def _render_template(options) #:nodoc:
+      lookup_context.rendered_format = nil if options[:formats]
       view_renderer.render(view_context, options)
     end
 

actionpack/test/controller/render_test.rb

@@ -549,6 +549,17 @@ def render_to_string_with_partial
     render :template => "test/hello_world"
   end
 
+  def render_to_string_with_template_and_html_partial
+    @text = render_to_string :template => "test/with_partial", :formats => [:text]
+    @html = render_to_string :template => "test/with_partial", :formats => [:html]
+    render :template => "test/with_html_partial"
+  end
+
+  def render_to_string_and_render_with_different_formats
+    @html = render_to_string :template => "test/with_partial", :formats => [:html]
+    render :template => "test/with_partial", :formats => [:text]
+  end
+
   def partial_with_counter
     render :partial => "counter", :locals => { :counter_counter => 5 }
   end
@@ -1263,6 +1274,21 @@ def test_render_to_string_partial
     assert_equal "text/html", @response.content_type
   end
 
+  def test_render_to_string_with_template_and_html_partial
+    get :render_to_string_with_template_and_html_partial
+    assert_equal "**only partial**\n", assigns(:text)
+    assert_equal "<strong>only partial</strong>\n", assigns(:html)
+    assert_equal "<strong>only html partial</strong>\n", @response.body
+    assert_equal "text/html", @response.content_type
+  end
+
+  def test_render_to_string_and_render_with_different_formats
+    get :render_to_string_and_render_with_different_formats
+    assert_equal "<strong>only partial</strong>\n", assigns(:html)
+    assert_equal "**only partial**\n", @response.body
+    assert_equal "text/plain", @response.content_type
+  end
+
   def test_partial_with_counter
     get :partial_with_counter
     assert_equal "5", @response.body

actionpack/test/fixtures/test/_partial_only_html.html

@@ -0,0 +1 @@
+only html partial

actionpack/test/fixtures/test/with_html_partial.html.erb

@@ -0,0 +1 @@
+<strong><%= render :partial => "partial_only_html" %></strong>

actionpack/test/fixtures/test/with_partial.html.erb

@@ -0,0 +1 @@
+<strong><%= render :partial => "partial_only" %></strong>

actionpack/test/fixtures/test/with_partial.text.erb

@@ -0,0 +1 @@
+**<%= render :partial => "partial_only" %>**