Description of problem:
When virt-who communicate with stage candlepin, if virt-who received 429, it will show "Error in communication with subscription manager" is cause by "TypeError: int() argument must be a string or a number, not 'NoneType'"

Version-Release number of selected component (if applicable):
virt-who-0.16-4.el6.noarch
subscription-manager-1.16.8-2.el6.x86_64
python-rhsm-1.16.6-1.el6.x86_64

How reproducible:
Always

Steps to Reproduce:
1.Register system to stage candlepin
2.Configure virt-who monitor esx and hyperv as the following:
[root@sgi-xe500-01 ~]# cat /etc/virt-who.d/virt
[test-esx1]
type=esx
server=10.73.2.95
username=Administrator
password=Welcome1!
owner=ACME_Corporation
env=Library
hypervisor_id=hostname
[root@sgi-xe500-01 ~]# cat /etc/sysconfig/virt-who  | grep -v ^# | grep -v ^$
VIRTWHO_DEBUG=1
VIRTWHO_INTERVAL=2
VIRTWHO_HYPERV=1
VIRTWHO_HYPERV_OWNER=ACME_Corporation
VIRTWHO_HYPERV_ENV=Library
VIRTWHO_HYPERV_SERVER=10.73.5.212
VIRTWHO_HYPERV_USERNAME=administrator
VIRTWHO_HYPERV_PASSWORD=Welcome1
3. Restart virt-who service and monitor virt-who's log
[root@sgi-xe500-01 ~]# service virt-who restart
[root@sgi-xe500-01 ~]# tail -f /var/log/rhsm/rhsm.log

Actual results:
When virt-who received 429 code, it will show error info as the following:
2016-02-25 03:08:33,202 [virtwho.main ERROR] MainProcess(1469):MainThread @virtwho.py:send:214 - Error in communication with subscription manager:
Traceback (most recent call last):
  File "/usr/share/virt-who/virtwho.py", line 197, in send
    self._sendGuestAssociation(report)
  File "/usr/share/virt-who/virtwho.py", line 224, in _sendGuestAssociation
    manager.hypervisorCheckIn(report, self.options)
  File "/usr/share/virt-who/manager/subscriptionmanager/subscriptionmanager.py", line 198, in hypervisorCheckIn
    retry_after = int(getattr(e, 'headers', {}).get('Retry-After'))
TypeError: int() argument must be a string or a number, not 'NoneType'
2016-02-25 03:08:33,202 [virtwho.main DEBUG] MainProcess(1469):MainThread @virtwho.py:send_current_report:166 - Report from "env/cmdline" failed to sent

Expected results:
It shouldn't show TypeError exception in virt-who log, Retry-After should get correct type.

Additional info:
Please see the detail error info in attachment virt-who.log