[exim] unwanted indentation at add_header headers

Hello,

I have an acl_smtp_data ACL, which looks like this:

warn    spam      = nobody:true
         message   = $spam_report

The spamd Server behind returns some additional headers, which I need
for later evaluation within exim. After the ACL is called, the added
header looks like this:

X-purgate: Clean.AlmostEmpty
    X-purgate-ID: 150741::080616222426-17C686C0-3F6C2A86/0-0/0-14
    X-purgate-Ad: For more information about eXpurgate please visit 
http://www.expurgate.net/

But I want it to look like this:

X-purgate: Clean.AlmostEmpty
X-purgate-ID: 150741::080616222426-17C686C0-3F6C2A86/0-0/0-14
X-purgate-Ad: For more information about eXpurgate please visit
http://www.expurgate.net/

How can I tell exim not to insert any indentations for this ACL?

I know that was an option for indentation within Received lines, but
while I like indentations in Received-headers, I don't like them in this
specific ACL.

Kind regards
Marten