allow types to be passed in for USING casts

This allows us so abstract the migration from the type that is actually
used by Rails.  For example, ":string" may be a varchar or something,
but the framework does that translation, and the app shouldn't need to
know.

activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb

@@ -435,6 +435,9 @@ def change_column(table_name, column_name, type, options = {})
           sql_type << "[]" if options[:array]
           sql = "ALTER TABLE #{quoted_table_name} ALTER COLUMN #{quote_column_name(column_name)} TYPE #{sql_type}"
           sql << " USING #{options[:using]}" if options[:using]
+          if options[:cast_as]
+            sql << " USING CAST(#{quote_column_name(column_name)} AS #{type_to_sql(options[:cast_as], options[:limit], options[:precision], options[:scale])})"
+          end
           execute sql
 
           change_column_default(table_name, column_name, options[:default]) if options_include_default?(options)