Merge branch 'patch-3' into 'stable-3.1'

Mulitple HSTS headers are not allowed, should either unset before adding new, or set conditionally.

See merge request ispconfig/ispconfig3!822

Author: Till Brehm

install/tpl/apache_ispconfig.vhost.master

@@ -88,7 +88,7 @@ NameVirtualHost *:<tmpl_var name="vhost_port">
   </tmpl_if>
 
   <IfModule mod_headers.c>
-    Header always add Strict-Transport-Security "max-age=15768000"
+    Header setifempty Strict-Transport-Security "max-age=15768000"
 	RequestHeader unset Proxy early
   </IfModule>