fix multipart upload: ignore additional headers

fix multipart upload: ignore additional headers

Reported in the forum:
https://forum.proxmox.com/threads/image-upload-fails-after-upgrading-from-7-1-to-7-3.119051/#post-516517

When additional headers existed in the request body, the upload failed.
With this patch, all additional headers get ignored.

Example: The following upload would fail because no headers were
expected after Content-Disposition.

```
--EPIHyQJFC5ftgoXHMe8-Jc6E7FqA4oMb0QBfOTz
Content-Disposition: form-data; name="content"
Content-Type: text/plain; charset=ISO-8859-1

iso
```
would fail. These headers now also get ignored, as we don't use them.

Also, upload now works when the Content-Disposition header isn't the
first, i.e.:
```
--XVH95dt1-A3J8mWiLCmHCW4roSC7-gBntjATBy--
Content-Type: text/plain; charset=ISO-8859-1
Content-Disposition: form-data; name="content"
```

Fixed upload was tested using
* Curl
* GUI
* Apache HttpClient 5

Signed-off-by: Matthias Heiserer <m.heiserer@proxmox.com>
Reviewed-by: Daniel Tschlatscher <d.tschlatscher@proxmox.com>
Tested-by:  Daniel Tschlatscher <d.tschlatscher@proxmox.com>