Normalize frames for trace items since they can contain invalid data.

Refs https://github.com/Seldaek/monolog/pull/474/files

The fix in the previous PR did not take into account that there might be object wrapped resources that would break json_encode, so the best solution would be normalizing those frames again.

@Seldaek Sorry for the inconvenience, but our graylog is still ramming up with those json_encode error messages.