ruby 2.3: fix marshalling problem

in ruby 2.1, AllowedServicesHash.reject {} will return a AllowedServicesHash,
on 2.3 it will return a plain Hash. so dup it instead (which returns an
AllowedServicesHash), and then reject! (which doesn't change the type
of the object). this works in both 2.1 and 2.3

Change-Id: Ic6eac219e31cd469afd45d026822654c11180b49
Reviewed-on: https://gerrit.instructure.com/92553
Tested-by: Jenkins
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

lib/account_services.rb

@@ -78,6 +78,8 @@ module AccountServices
   end
 
   def self.default_allowable_services
-    self.allowable_services.reject {|_, info| info[:default] == false }
+    res = self.allowable_services.dup
+    res.reject! {|_, info| info[:default] == false }
+    res
   end
 end