Add a check to ensure created_at and updated_at are Time

[kalinon]

If a column is named created_at or updated_at is not a type Time?, then the following occurs:

Code in lib/granite/src/granite/transactions.cr:82:5

 82 | {% if @type.instance_vars.select { |ivar| ivar.annotation(Granite::Column) }.map(&.name.stringify).includes? "created_at" %}
      ^
Called macro defined in lib/granite/src/granite/transactions.cr:82:5

 82 | {% if @type.instance_vars.select { |ivar| ivar.annotation(Granite::Column) }.map(&.name.stringify).includes? "created_at" %}

Which expanded to:

 > 1 |
 > 2 |       if mode == :create
 > 3 |         @created_at = time.at_beginning_of_second
                                  ^---------------------

this simply adds a type check.