xhtml_style: don't set content type header if already set

This is necessary on Zope4 because the error page is executed even if
the response already has a locked body - and in this case we don't
want to override the content-type header that might also have been set.

One user of this is Base_redirect from erp5_hal_json_style, when
abort_transaction is True, in that case it sets a body in json, a
content type header of application/json and then raise a redirect so
that the transaction is not commited. This fixes a problem visible on
Zope4 with erp5_bank_reconciliation_renderjs_ui_test:testFunctionalRJSBankReconciliationAction

product/ERP5/bootstrap/erp5_xhtml_style/SkinTemplateItem/portal_skins/erp5_xhtml_style/global_definitions.zpt

@@ -16,7 +16,7 @@
            global current_action   python: portal.ERP5Site_getCurrentAction(current_url, actions);
            cancel_url       request/cancel_url | current_url;
            dummy            python: local_parameter_list.update({'cancel_url': cancel_url});
-           dummy       python: request.RESPONSE.setHeader('Content-Type', 'text/html;; charset=utf-8');
+           dummy       python: request.RESPONSE.getHeader('Content-Type') or request.RESPONSE.setHeader('Content-Type', 'text/html;; charset=utf-8');
            dummy  python: request.set('here', here);
            global css_list  css_list | python:[];
            dummy       python:css_list.extend(['%s/%s' % (portal_path, x) for x in portal.ERP5Site_getCssRelativeUrlList()]);