Rack body respond to each and not to join

This fixes undef `to_str' for Rack::Chunked::Body when using
caches_action + streaming on an action

Closes #5027

actionpack/lib/action_controller/caching/actions.rb

@@ -103,8 +103,10 @@ def caches_action(*actions)
       end
 
       def _save_fragment(name, options)
-        content = response_body
-        content = content.join if content.is_a?(Array)
+        content = ""
+        response_body.each do |parts|
+          content << parts
+        end
 
         if caching_allowed?
           write_fragment(name, content, options)

actionpack/test/controller/caching_test.rb

@@ -237,6 +237,7 @@ class ActionCachingTestController < CachingController
   caches_action :with_format_and_http_param, :cache_path => Proc.new { |c| { :key => 'value' } }
   caches_action :layout_false, :layout => false
   caches_action :record_not_found, :four_oh_four, :simple_runtime_error
+  caches_action :streaming
 
   layout 'talk_from_action'
 
@@ -296,6 +297,10 @@ def expire_with_url_string
     expire_action url_for(:controller => 'action_caching_test', :action => 'index')
     render :nothing => true
   end
+
+  def streaming
+    render :text => "streaming", :stream => true
+  end
 end
 
 class MockTime < Time
@@ -647,6 +652,13 @@ def test_simple_runtime_error_returns_500_for_multiple_requests
     assert_response 500
   end
 
+  def test_action_caching_plus_streaming
+    get :streaming
+    assert_response :success
+    assert_match(/streaming/, @response.body)
+    assert fragment_exist?('hostname.com/action_caching_test/streaming')
+  end
+
   private
     def content_to_cache
       assigns(:cache_this)