ASP.NET: Posting form--post to relative vs. absolute action

Question

I've got an ASP.NET form on an HTTP:// page with method="post". If I set the "action" attribute to an absolute HTTPS URL at runtime and submit the form using JavaScript, it will perform a GET for the action URL (I'm watching with Fiddler). However, if I use a relative path as the "action", the form POSTs like it should when I submit.

What's going on here? I need to use an absolute URL because I need to be posting to HTTPS whereas the pages with the form are HTTP.

Here's the JS (I'm using MooTools):

form.set('action', '/Test/Test').submit(); // performs POST
form.set('action', 'https://.../Test/Test').submit() // performs GET
form.set('action', 'https://.../Test/Test').set('method', 'post').submit() // performs GET