Fix tests on 1.8 by explicitly checking for strings (which also improves performance).

actionpack/lib/action_dispatch/http/response.rb

@@ -118,7 +118,15 @@ def body
 
     def body=(body)
       @blank = true if body == EMPTY
-      @body = body.respond_to?(:each) ? body : [body]
+
+      # Explicitly check for strings. This is *wrong* theoretically
+      # but if we don't check this, the performance on string bodies
+      # is bad on Ruby 1.8 (because strings responds to each then).
+      @body = if body.respond_to?(:to_str) || !body.respond_to?(:each)
+        [body]
+      else
+        body
+      end
     end
 
     def body_parts