Use ActiveRecord Column#type_cast to properly parse fields represented by a string.

Fixed issued reported by Nick Burdick where imported datetime fields was not correctly working with certain string formats for MySQL which doesn't support MM/DD/YYYY format directly.

lib/activerecord-import/adapters/abstract_adapter.rb

@@ -129,7 +129,8 @@ module ActiveRecord::Import::AbstractAdapter
       array_of_attributes.each do |arr|
         my_values = []
         arr.each_with_index do |val,j|
-          my_values << quote( val, columns[j] )
+          importable_value = columns[j].type_cast(val)
+          my_values << quote(importable_value, columns[j] )
         end
         values << my_values
       end