[HttpCompressionMiddleware] fix delimiter for Accept-Encoding header
diff --git a/scrapy/downloadermiddlewares/httpcompression.py b/scrapy/downloadermiddlewares/httpcompression.py
index 65b65295365..0010b2a8f2a 100644
--- a/scrapy/downloadermiddlewares/httpcompression.py
+++ b/scrapy/downloadermiddlewares/httpcompression.py
@@ -26,7 +26,7 @@ def from_crawler(cls, crawler):
 
     def process_request(self, request, spider):
         request.headers.setdefault('Accept-Encoding',
-                                   b",".join(ACCEPTED_ENCODINGS))
+                                   b", ".join(ACCEPTED_ENCODINGS))
 
     def process_response(self, request, response, spider):
 
diff --git a/tests/test_downloadermiddleware_httpcompression.py b/tests/test_downloadermiddleware_httpcompression.py
index c6a823b535c..64488841a29 100644
--- a/tests/test_downloadermiddleware_httpcompression.py
+++ b/tests/test_downloadermiddleware_httpcompression.py
@@ -48,7 +48,7 @@ def _getresponse(self, coding):
                 }
 
         response = Response('http://scrapytest.org/', body=body, headers=headers)
-        response.request = Request('http://scrapytest.org', headers={'Accept-Encoding': 'gzip,deflate'})
+        response.request = Request('http://scrapytest.org', headers={'Accept-Encoding': 'gzip, deflate'})
         return response
 
     def test_process_request(self):
@@ -56,7 +56,7 @@ def test_process_request(self):
         assert 'Accept-Encoding' not in request.headers
         self.mw.process_request(request, self.spider)
         self.assertEqual(request.headers.get('Accept-Encoding'),
-                         b','.join(ACCEPTED_ENCODINGS))
+                         b', '.join(ACCEPTED_ENCODINGS))
 
     def test_process_response_gzip(self):
         response = self._getresponse('gzip')

Accept-Encoding header should have `, `  (with space) as a delimiter between encodings instead of `,` (non space) (https://tools.ietf.org/html/rfc2616#section-14.3)