handle UTF-16 error on ruby 2.2

Change-Id: I4fff1e2963d2a5eb3d544b546b2eada0f4b106b2
Reviewed-on: https://gerrit.instructure.com/56058
Tested-by: Jenkins
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

lib/canvas/migration/xml_helper.rb

@@ -100,7 +100,12 @@ module XMLHelper
     if doc.encoding != 'UTF-8'
       begin
         doc.at_css('*')
+      rescue ArgumentError => e
+        # ruby 2.2
+        raise unless e.message =~ /^invalid byte sequence/
+        doc.encoding = 'UTF-8'
       rescue Encoding::CompatibilityError
+        # ruby 2.1
         doc.encoding = 'UTF-8'
       end
     end