diff --git a/scrapy/core/downloader/handlers/http11.py b/scrapy/core/downloader/handlers/http11.py
index 37e836809..55bd31303 100644
--- a/scrapy/core/downloader/handlers/http11.py
+++ b/scrapy/core/downloader/handlers/http11.py
@@ -279,8 +279,7 @@ class ScrapyAgent(object):
             headers.removeHeader(b'Proxy-Authorization')
         if request.body:
             bodyproducer = _RequestBodyProducer(request.body)
-        else:
-            bodyproducer = None
+        elif method == b'POST':
             # Setting Content-Length: 0 even for POST requests is not a
             # MUST per HTTP RFCs, but it's common behavior, and some
             # servers require this, otherwise returning HTTP 411 Length required
@@ -289,10 +288,13 @@ class ScrapyAgent(object):
             # "a Content-Length header field is normally sent in a POST
             # request even when the value is 0 (indicating an empty payload body)."
             #
-            # Twisted Agent will not add "Content-Length: 0" by itself
-            if method == b'POST':
-                headers.addRawHeader(b'Content-Length', b'0')
-
+            # Twisted < 17 will not add "Content-Length: 0" by itself;
+            # Twisted >= 17 fixes this;
+            # Using a producer with an empty-string sends `0` as Content-Length
+            # for all versions of Twisted.
+            bodyproducer = _RequestBodyProducer(b'')
+        else:
+            bodyproducer = None
         start_time = time()
         d = agent.request(
             method, to_bytes(url, encoding='ascii'), headers, bodyproducer)
diff --git a/tests/mockserver.py b/tests/mockserver.py
index 26ab51183..b95a6c3c4 100644
--- a/tests/mockserver.py
+++ b/tests/mockserver.py
@@ -15,7 +15,6 @@ from twisted.internet.task import deferLater
 
 
 from scrapy.utils.python import to_bytes, to_unicode
-from tests import tests_datadir
 
 
 def getarg(request, name, default=None, type=None):
@@ -122,6 +121,7 @@ class Echo(LeafResource):
             'body': to_unicode(request.content.read()),
         }
         return to_bytes(json.dumps(output))
+    render_POST = render_GET
 
 
 class RedirectTo(LeafResource):
@@ -174,7 +174,11 @@ class Root(Resource):
         self.putChild(b"echo", Echo())
         self.putChild(b"payload", PayloadResource())
         self.putChild(b"xpayload", EncodingResourceWrapper(PayloadResource(), [GzipEncoderFactory()]))
-        self.putChild(b"files", File(os.path.join(tests_datadir, 'test_site/files/')))
+        try:
+            from tests import tests_datadir
+            self.putChild(b"files", File(os.path.join(tests_datadir, 'test_site/files/')))
+        except:
+            pass
         self.putChild(b"redirect-to", RedirectTo())
 
     def getChild(self, name, request):
diff --git a/tests/test_downloader_handlers.py b/tests/test_downloader_handlers.py
index 3efcf6e9c..0f28037ba 100644
--- a/tests/test_downloader_handlers.py
+++ b/tests/test_downloader_handlers.py
@@ -29,7 +29,7 @@ from scrapy.core.downloader.handlers.http11 import HTTP11DownloadHandler
 from scrapy.core.downloader.handlers.s3 import S3DownloadHandler
 
 from scrapy.spiders import Spider
-from scrapy.http import Request
+from scrapy.http import Headers, Request
 from scrapy.http.response.text import TextResponse
 from scrapy.responsetypes import responsetypes
 from scrapy.settings import Settings
@@ -37,7 +37,7 @@ from scrapy.utils.test import get_crawler, skip_if_no_boto
 from scrapy.utils.python import to_bytes
 from scrapy.exceptions import NotConfigured
 
-from tests.mockserver import MockServer, ssl_context_factory
+from tests.mockserver import MockServer, ssl_context_factory, Echo
 from tests.spiders import SingleRequestSpider
 
 class DummyDH(object):
@@ -202,6 +202,7 @@ class HttpTestCase(unittest.TestCase):
         r.putChild(b"broken-chunked", BrokenChunkedResource())
         r.putChild(b"contentlength", ContentLengthHeaderResource())
         r.putChild(b"nocontenttype", EmptyContentTypeHeaderResource())
+        r.putChild(b"echo", Echo())
         self.site = server.Site(r, timeout=None)
         self.wrapper = WrappingFactory(self.site)
         self.host = 'localhost'
@@ -310,6 +311,17 @@ class HttpTestCase(unittest.TestCase):
         request = Request(self.getURL('contentlength'), method='POST', headers={'Host': 'example.com'})
         return self.download_request(request, Spider('foo')).addCallback(_test)
 
+    def test_content_length_zero_bodyless_post_only_one(self):
+        def _test(response):
+            import json
+            headers = Headers(json.loads(response.text)['headers'])
+            contentlengths = headers.getlist('Content-Length')
+            self.assertEquals(len(contentlengths), 1)
+            self.assertEquals(contentlengths, [b"0"])
+
+        request = Request(self.getURL('echo'), method='POST')
+        return self.download_request(request, Spider('foo')).addCallback(_test)
+
     def test_payload(self):
         body = b'1'*100 # PayloadResource requires body length to be 100
         request = Request(self.getURL('payload'), method='POST', body=body)