Avoid Sending Regular Content-Length header for "304 Not Modified" responses

[alanfairless]

This causes web_public_reader to issue a 500 error when running unit test TestModified.test_modification_restriction:

From the web public reader log:

10.0.3.14 - - [2014-02014-09-28 02:53:17,389 INFO     Application         : start request 18269300-e1aa-44bf-b246-e0c980e079ca: 'HEAD' 'http://rr-motoboto-test-01-20140928025317286950.nimbus.io/data/test-key.jpg'
2014-09-28 02:53:17,394 INFO     Application         : request 18269300-e1aa-44bf-b246-e0c980e079ca: head_key: collection = (331) rr-motoboto-test-01-20140928025317286950 key = test-key.jpg None
2014-09-28 02:53:17,403 ERROR    Application         : request 18269300-e1aa-44bf-b246-e0c980e079ca HTTPNotFound Not Found: u'test-key.jpg' head-key 'http://rr-motoboto-test-01-20140928025317286950.nimbus.io/data/test-key.jpg'
2014-09-28 02:53:19,069 INFO     Application         : start request eaa9977c-f43e-4345-a7c4-f15ac9f02710: 'HEAD' 'http://rr-motoboto-test-01-20140928025317286950.nimbus.io/data/test-key.jpg'
2014-09-28 02:53:19,070 INFO     Application         : request eaa9977c-f43e-4345-a7c4-f15ac9f02710: head_key: collection = (331) rr-motoboto-test-01-20140928025317286950 key = test-key.jpg None
2014-09-28 02:53:19,076 INFO     Application         : response to request eaa9977c-f43e-4345-a7c4-f15ac9f02710: dispatch status_int=200 app_iter=True
2014-09-28 02:53:20,170 INFO     Application         : start request 89124d66-0ca2-4673-a56d-db6ea67b4031: 'HEAD' 'http://rr-motoboto-test-01-20140928025317286950.nimbus.io/data/test-key.jpg'
2014-09-28 02:53:20,172 INFO     Application         : request 89124d66-0ca2-4673-a56d-db6ea67b4031: head_key: collection = (331) rr-motoboto-test-01-20140928025317286950 key = test-key.jpg None
2014-09-28 02:53:20,188 INFO     Application         : response to request 89124d66-0ca2-4673-a56d-db6ea67b4031: dispatch status_int=200 app_iter=True
2014-09-28 02:53:20,271 INFO     Application         : start request d67e1520-be33-4d23-be59-3ebec0b05f35: 'HEAD' 'http://rr-motoboto-test-01-20140928025317286950.nimbus.io/data/test-key.jpg'
2014-09-28 02:53:20,274 INFO     Application         : request d67e1520-be33-4d23-be59-3ebec0b05f35: head_key: collection = (331) rr-motoboto-test-01-20140928025317286950 key = test-key.jpg None
2014-09-28 02:53:20,283 INFO     Application         : response to request d67e1520-be33-4d23-be59-3ebec0b05f35: dispatch status_int=304 app_iter=True
Traceback (most recent call last):
  File "gevent/pywsgi.py", line 508, in handle_one_response
    self.run_application()
  File "gevent/pywsgi.py", line 494, in run_application
    self.result = self.application(self.environ, self.start_response)
  File "webob/dec.py", line 159, in __call__
    return resp(environ, start_response)
  File "webob/response.py", line 939, in __call__
    start_response(self.status, headerlist)
  File "gevent/pywsgi.py", line 458, in start_response
    raise AssertionError(msg)
AssertionError: Invalid Content-Length for 304 response: '1048576' (must be absent or zero)
{'GATEWAY_INTERFACE': 'CGI/1.1',
 'HTTP_ACCEPT_ENCODING': 'identity',
 'HTTP_AGENT': 'lumberyard/1.0',
 'HTTP_AUTHORIZATION': 'NIMBUS.IO 1:3ab72050b33fc8ec55900cef4e844580943d4fac9fc7ac48057733bfaeb23c55',
 'HTTP_HOST': 'rr-motoboto-test-01-20140928025317286950.nimbus.io',
 'HTTP_IF_MODIFIED_SINCE': 'Sun, 28 Sep 2014 02:53:20 GMT',
 'HTTP_USER_AGENT': 'Go 1.1 package http',
 'HTTP_X_FORWARDED_FOR': '10.0.3.6:47818',
 'HTTP_X_NIMBUS_IO_TIMESTAMP': '1411872800',
 'HTTP_X_NIMBUS_IO_USER_REQUEST_ID': 'd67e1520-be33-4d23-be59-3ebec0b05f35',
 'PATH_INFO': '/data/test-key.jpg',
 'QUERY_STRING': '', 
 'REMOTE_ADDR': '10.0.3.14',
 'REMOTE_PORT': '40669',
 'REQUEST_METHOD': 'HEAD',
 'SCRIPT_NAME': '', 
 'SERVER_NAME': 'sim-ash10',
 'SERVER_PORT': '9000',
 'SERVER_PROTOCOL': 'HTTP/1.1',
 'SERVER_SOFTWARE': 'gevent/1.0 Python/2.7',
 'webob._parsed_query_vars': (GET([]), ''),
 'webob.adhoc_attrs': {'response': <Response at 0x279eed0 200 OK>},
 'wsgi.errors': <open file '<stderr>', mode 'w' at 0x7fa4e93bd1e0>,
 'wsgi.input': <gevent.pywsgi.Input object at 0x270ce90>,
 'wsgi.multiprocess': False,
 'wsgi.multithread': False,
 'wsgi.run_once': False,
 'wsgi.url_scheme': 'http',
 'wsgi.version': (1, 0)} failed with AssertionError
2014-09-28T02:53:20.29015 

[dougfort]

fc6f590