Form: set response content type when called directly

Form.__call__ replaces page template call, so it is also responsible
to set the content type in response.

With Page Templates, ZopePageTemplate._exec (called by
Shared.DC.Scripts.Bindings.__call__ and _bindAndExec) sets the
response content type and calls pt_render. Form.__call__ also calls
pt_render, so to behave like page template, it should also sets the
response content type.

In erp5_xhtml_style, we don't notice this, because global_definitions
contains a setHeader call to set content type to html.