NetworkPkg/HttpBootDxe: Update to check specified media type

NetworkPkg/HttpBootDxe: Update to check specified media type

IANA has approved below new media type for EFI http(s) boot usage:
  application/vnd.efi.img
  application/vnd.efi.iso

HTTP boot driver should be updated to check the above media type
from Content-Type header field.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>