Adding delsp=yes to Content-Type

[sunnycemetery]

Hello, all.  I tried inserting the following MIME header from my editor:
Content-Type: text/plain; charset=utf-8; format=flowed; delsp=yes;

But mutt ignores the delsp parameter (and possibly the entire header 
declaration, as well).  The $edit_headers option is set.  (I regularly 
use “Attach”, “In-Reply-To”, &c.)

So how do I set delsp=yes from mutt?  Thank you.