Don't cache the utc_offset we are already caching the timezone [#4301]

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

activesupport/lib/active_support/values/time_zone.rb

@@ -181,10 +181,12 @@ def initialize(name, utc_offset, tzinfo = nil)
       @name = name
       @utc_offset = utc_offset
       @tzinfo = tzinfo
+      @current_period = nil
     end
 
     def utc_offset
-      @utc_offset ||= tzinfo.current_period.utc_offset
+      @current_period ||= tzinfo.current_period
+      @current_period.utc_offset
     end
 
     # Returns the offset of this time zone as a formatted string, of the