I think it depends on whether we want to talk about these as ``headers`` or as additional params.  If we use the term ``headers`` I kind of feel that it makes sense to make them look like headers, e.g. ``content-type: text/plain``.  If, on the other hand, we want to treat them additional parameters that haven't been broken out separately and just coincidentally align to HTTP headers, then we should use the parameter name, e.g. ``ContentType=text/plain``.

I think I'm preferring the later (i.e. params).  Preferences?