Small testcase:

$.post('/index')

no Content-Type and Content-Length headers

$.post('/index', null)

no Content-Type and Content-Length headers

$.post('/index', {'test': 'test'})

Content-Type	application/x-www-form-urlencoded
Content-Length	9

$.post('/index', {})

Content-Length	0
Content-Type	application/xml

$.post('/index', function () {})

Content-Length	0
Content-Type	application/xml

It's wrong Content-type application/xml in last two exaples.

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.14) Gecko/20080419 Ubuntu/8.04 (hardy) Firefox/2.0.0.14