Issue 2756 - urllib2 add_header fails with existing unredirected header

The issue noticable here:

[ors@goofy ~]$ python
Python 2.6b2+ (trunk:65482M, Aug 4 2008, 14:26:01)
[GCC 3.4.2 20041017 (Red Hat 3.4.2-6.fc3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> url = 'http://hroch486.icpf.cas.cz/formpost.html'
>>> import urllib2
>>> req_obj = urllib2.Request(url)
>>> req_obj.unredirected_hdrs
{}
>>> req_obj.add_data("Spam")
>>> req_obj.unredirected_hdrs
{}
>>> response = urllib2.urlopen(req_obj)
>>> req_obj.unredirected_hdrs
{'Content-length': '4', 'Content-type': 'application/x-www-form-urlencoded',
'Host': 'hroch486.icpf.cas.cz', 'User-agent': 'Python-urllib/2.6'}
>>> req_obj.add_data("SpamBar")
>>> req_obj.add_header("Content-type","application/html")
>>> response = urllib2.urlopen(req_obj)
>>> req_obj.unredirected_hdrs
{'Content-length': '4', 'Content-type': 'application/x-www-form-urlencoded',
'Host': 'hroch486.icpf.cas.cz', 'User-agent': 'Python-urllib/2.6'}
>>> req_obj.get_data()
'SpamBar'
>>>

In the final req_obj call, the unredirected_hdrs had not changed in neither
Content-length nor in Content-type.