Regarding the original bug reported by Cocobear, I think this is by design. The “addheaders” attribute seems to serve as a set of default header fields, although this is hardly obvious from the documentation. In fact the current documentation suggests you have to use the application/x-www-form-urlencoded content type for request data. I think the documentation should be relaxed; I opened Issue 23360 about that.

The rest of the thread seems to have side-tracked into patches to make Request.has_header() and friends case insensitive. I’ll have to study the threads closer, but I think this side-track is a duplicate of Issue 2275.