Time#as_json: use Time#formatted_offset instead of strftime %z direct…

…ive, which is non-standard and inaccurate on some platforms (e.g., Mac OS X).

[#4979]

Signed-off-by: Jeremy Kemper <jeremy@bitsweat.net>

activesupport/lib/active_support/json/encoding.rb

@@ -212,7 +212,7 @@ def as_json(options = nil) #:nodoc:
     if ActiveSupport.use_standard_json_time_format
       xmlschema
     else
-      strftime("%Y/%m/%d %H:%M:%S %z")
+      %(#{strftime("%Y/%m/%d %H:%M:%S")} #{formatted_offset(false)})
     end
   end
 end