nebraska: Fix request.headers.getheader() for python3

Python 3 does't have request.headers.getheader() anymore. It is
replaced by  request.headers.get() (request.headers is a dict now).
This has caused cros flash to fail.

This change seems to work with python2 too.

BUG=chromium:1029201
TEST=cros flash on a VM betty image and reef
TEST=nebraska_unittests

Change-Id: I7a68a43697252eb566e51275460b9548a870a55f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1941467
Commit-Queue: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Auto-Submit: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/nebraska/nebraska.py b/nebraska/nebraska.py
index 1e20eff..2d2eb0c 100755
--- a/nebraska/nebraska.py
+++ b/nebraska/nebraska.py
@@ -808,9 +808,10 @@
           no update (even if there is).
       """
       try:
-        request_len = int(self.headers.getheader('content-length'))
+        request_len = int(self.headers.get('content-length'))
         request = self.rfile.read(request_len)
       except Exception as err:
+        logging.error('Failed to read request in do_POST %s', str(err))
         self.send_error(http_client.BAD_REQUEST, 'Invalid request (header).')
         return