Fix for read_multi not returning deserialized values and potential marshalling issues

read_multi and marshalling were causing me some issues.

Using ActiveSupport::Cache::FileStore

	Loading development environment (Rails 3.2.13)
	irb(main):001:0> cache = Rails.cache
	=> #<ActiveSupport::Cache::FileStore:0x007fca7c0b65f0 @options={}, ...
	irb(main):002:0> cache.fetch('testing') { { color: "green" } }
	=> {:color=>"green"}
	irb(main):003:0> cache.read('testing')
	=> {:color=>"green"}
	irb(main):004:0> cache.read_multi('testing')
	=> {"testing"=>{:color=>"green"}}
	irb(main):005:0>

Using ActiveSupport::Cache::RedisStore.new

	Loading development environment (Rails 3.2.13)
	irb(main):001:0> cache = ActiveSupport::Cache::RedisStore.new
	=> #<ActiveSupport::Cache::RedisStore:0x007f9c761413f0 @data=#<Redis client v3.0.4 for redis://127.0.0.1:6379/0>, @options={}>
	irb(main):002:0> cache.fetch('testing') { { color: "green" } }
	=> {:color=>"green"}
	irb(main):003:0> cache.read('testing')
	=> {:color=>"green"}
	irb(main):004:0> cache.read_multi('testing')
	=> {"testing"=>#<ActiveSupport::Cache::Entry:0x007f9c78588d58 @compressed=false, @expires_in=nil, @created_at=1369021269.372503, @value="\x04\b{\x06:\ncolorI\"\ngreen\x06:\x06ET">}
	irb(main):005:0>