TT#21050 Tighten POST checks

* Validate Content-Type before validating content itself.
* Validate Prefer

Change-Id: Ie966fa5f0df0b8b695130ea5e4384ff84773bd87

lib/NGCP/Panel/Role/API.pm

@@ -86,7 +86,10 @@ sub get_valid_post_data {
     my $c = $params{c};
     my $media_type = $params{media_type};
     my $json =  $self->get_valid_raw_post_data(%params);
+    return unless $self->valid_media_type($c, $media_type);
+    return unless $self->require_preference($c);
     return unless $self->require_wellformed_json($c, $media_type, $json);
+
     return JSON::from_json($json, { utf8 => 1 });
 }