Do not strip Content-Type header for POST requests to swift

libcurl adds 'Content-Type: application/x-www-form-urlencoded'
to a POST request if the 'Content-Type' header is not set
manually. Because data in swift is updated via POST, the
Content-Type header must be set explicitly to stop a run of
refreshFileHeaders.php from changing the Content-Type of all
files in swift to application/x-www-form-urlencoded

Bug: T178849
Change-Id: I43c21bc1b73e37104cf07cd5f1c1557f472b9898