Include Calculations module instead of prepending (#928)

Another gem (groupdate) also patches the #calculate method
and prepending this module prevents any other libraries from
patching this method.

Author: ehutzelman

lib/active_record/connection_adapters/sqlserver/core_ext/calculations.rb

@@ -43,5 +43,5 @@ def build_count_subquery(relation, column_name, distinct)
 
 ActiveSupport.on_load(:active_record) do
   mod = ActiveRecord::ConnectionAdapters::SQLServer::CoreExt::Calculations
-  ActiveRecord::Relation.prepend(mod)
+  ActiveRecord::Relation.include(mod)
 end