handle <%== nil %> cases

This is much less common than string literal appends, so add a special
case method for it.  Maybe fixes bug reported by @jeremy on 97ef636191

actionview/lib/action_view/buffers.rb

@@ -13,6 +13,11 @@ module ActionView
     end
     alias :append= :<<
 
+    def safe_expr_append=(val)
+      return self if val.nil?
+      safe_concat val.to_s
+    end
+
     alias :safe_append= :safe_concat
   end
 

actionview/lib/action_view/template/handlers/erb.rb

@@ -49,9 +49,9 @@ module ActionView
         def add_expr_escaped(src, code)
           flush_newline_if_pending(src)
           if code =~ BLOCK_EXPR
-            src << "@output_buffer.safe_append= " << code
+            src << "@output_buffer.safe_expr_append= " << code
           else
-            src << "@output_buffer.safe_append=(" << code << ");"
+            src << "@output_buffer.safe_expr_append=(" << code << ");"
           end
         end
 

actionview/test/fixtures/test/nil_return.erb

@@ -0,0 +1 @@
+This is nil: <%== nil %>

actionview/test/template/compiled_templates_test.rb

@@ -5,6 +5,10 @@ class CompiledTemplatesTest < ActiveSupport::TestCase
     ActionView::LookupContext::DetailsKey.clear
   end
 
+  def test_template_with_nil_erb_return
+    assert_equal "This is nil: \n", render(:template => "test/nil_return")
+  end
+
   def test_template_gets_recompiled_when_using_different_keys_in_local_assigns
     assert_equal "one", render(:file => "test/render_file_with_locals_and_default")
     assert_equal "two", render(:file => "test/render_file_with_locals_and_default", :locals => { :secret => "two" })