Added test for Array#to_formatted_s

Behavior different from the contents currently written to rdoc is carried out. 

> Blog.all.map(&:title) #=> ["First Post", "Second Post", "Third post"]
>
> `to_formatted_s` shows us:
>
>   Blog.all.to_formatted_s # => "First PostSecond PostThird Post"

But, wrote test code is fails.

>  1) Failure:
> ArrayExtToFormattedS#test_to_formatted_default [/Users/eiel/src/rails/activesupp
> ort/test/core_ext/array_ext_test.rb:41]:
> --- expected
> +++ actual
> @@ -1 +1 @@
> -"First ObjectSecond ObjectThird Object"
> +"[#<#<Class:0xXXXXXX>:0xXXXXXX>, #<#<Class:0xXXXXXX>:0xXXXXXX>, #<#<Class:0xXXXXXX>:0xXXXXXX>]"

As the simplistic solution thought of: 
```diff
diff --git a/activesupport/lib/active_support/core_ext/array/conversions.rb b/activesupport/lib/active_support/core_ext/array/conversions.rb
index 430a35f..500cb73 100644
--- a/activesupport/lib/active_support/core_ext/array/conversions.rb
+++ b/activesupport/lib/active_support/core_ext/array/conversions.rb
@@ -108,7 +108,7 @@ class Array
         collect { |element| element.id }.join(',')
       end
     else
-      to_default_s
+      collect { |element| element.to_s }.join
     end
   end
   alias_method :to_default_s, :to_s
```

Influence seems to be great to others. 
First of all, please let me know what kind of behavior is right.