Nova should not log 'Unrecognized content-type...' when none is provided

Fix proposed to branch: master
Review: https://review.openstack.org/3107

What's happening here is that nova always gets 'text/plain' if we don't explicitly provide one. So it always logs an 'Unrecognized...' when really it should not be getting a content type at all and log a 'No content-type provided' line.

I've seen this message act as a time-wasting red herring for people trying to debug an unrelated issue.

Created a FAQ for the issue: https://answers.launchpad.net/nova/+faq/2003

Fix proposed to branch: master
Review: https://review.openstack.org/10119

Reviewed: https://review.openstack.org/10119
Committed: http://github.com/openstack/nova/commit/9ac962cc82ef55cf211c0dfa5d18bc5bde27563b
Submitter: Jenkins
Branch: master

commit 9ac962cc82ef55cf211c0dfa5d18bc5bde27563b
Author: Mark McLoughlin <email address hidden>
Date: Mon Jul 23 09:32:30 2012 +0100

    Avoid unrecognized content-type message

    Fixes bug #904473

    If the client does not include a Content-type header in its request
    (which is normal for e.g. GET and DELETE) then eventlet will pass us
    'text/plain'.

    Because 'text/plain' isn't one of our supported request content types
    we log:

      Unrecognized Content-Type provided in request

    This can be an annoying red-herring for people debugging issues.

    The reason that eventlet defaults to text/plain is because it uses
    the mimetools.Message.gettype() method:

      Message.gettype()
        Return the message type [..] as specified in the Content-Type
        header. If no such header exists, return 'text/plain'.

    The wsgiref web server has the same behavior, whereas mod_wsgi will
    pass no content-type and twisted will pass an empty string.

    Make our code more robust by treating the empty string or text/plain
    as if no content-type header was supplied.

    Change-Id: Ide117c807db0dc0f5cbe974788b604b5e236800a