Do not use HashWithIndifferentAccess class unless it defined

Change-Id: I703c5a3ed784eb8e2b1a1ca5d58c898951e1393d
Reviewed-on: http://review.couchbase.org/21773
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>

lib/couchbase/model.rb

@@ -425,8 +425,8 @@ def self.create(*args)
     def initialize(attrs = {})
       @errors = ::ActiveModel::Errors.new(self) if defined?(::ActiveModel)
       case attrs
-      when Hash, HashWithIndifferentAccess
-        if attrs.respond_to?(:with_indifferent_access)
+      when Hash
+        if defined?(HashWithIndifferentAccess) && !attrs.is_a?(HashWithIndifferentAccess)
           attrs = attrs.with_indifferent_access
         end
         @id = attrs.delete(:id)