From the docs of the Navigate method:

Headers

Optional. A value that specifies additional HTTP headers to send to the server. These headers are added to the default Microsoft Internet Explorer headers. The headers can specify things like the action required of the server, the type of data being passed to the server, or a status code. This parameter is ignored if URL is not an HTTPURL.

This probably implies, that a

Content-Type: application/x-www-form-urlencoded
[download]

has to be passed to make it a POST.

This is also noted in the the article ikegami refered to as providing a workaround, which then says that "It will work on some sites, but on many others you'll get an error."

/me scratches head