Fix average calculation in ruby 2.6

Ruby 2.6 added `to_d` to BigDecimal and it was causing some tests
to fail because of that.

See rails/rails#34858

Author: wpolicarpo

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

@@ -11,6 +11,15 @@ module Calculations
           def build_count_subquery(relation, column_name, distinct)
             super(relation.unscope(:order), column_name, distinct)
           end
+
+          def type_cast_calculated_value(value, type, operation = nil)
+            case operation
+            when "count"   then value.to_i
+            when "sum"     then type.deserialize(value || 0)
+            when "average" then value&.respond_to?(:to_d) ? value.to_d : value
+            else type.deserialize(value)
+            end
+          end
         end
       end
     end