Http headers

I was looking into a bug that keeps a popup open in the backend of Formbuilder. When you save a field, the ajax call returns a 500 internal server error. The error_log file: `error parsing headers: duplicate header 'content-type'`

It's caused by the template.php file when the display function is called twice. I can't set the [$customHeaders](https://github.com/forkcms/forkcms/blob/master/backend/core/engine/template.php#L75) variable because [save_field](https://github.com/forkcms/forkcms/blob/master/backend/modules/form_builder/engine/model.php#L685) calls [getContent](https://github.com/forkcms/library/blob/master/spoon/template/template.php#L406) from Spoon.

Is this the best place to add the charset? Better solutions? Thoughts?

Thx @dubgeiser and @dieterve for the help