Fix mangled times after save when default_timezone is set to local

[mdotterer]

Time attributes are being mangled in the Active Record after filter changes_internally_applied because the new attribute is being based of the serialized version of the time, which is a string. The new column value comes from parsing this string, but the old parsing method does not account for the default_timezone setting causing the the time to be mis-interpreted.

[metaskills]

Thanks! I really like your implementation of the fast_string_to_time method. The tests concern me and feel out of place. It strikes me as odd that somehow some of ActiveRecord's BasicsTest did not fail without this fix. Specifically the test_preserving_time_objects_with_local_time_conversion_to_default_timezone_utc test. Going to merge this in tho. Again, thanks!