serve: don't send any content headers with 304 responses

Fixes HTTP protocol violation introduced in cf5c76017e11. 'hg serve' would show
a stacktrace when loading pages that not had been modified.

There was test coverage for this, but the wrong response headers wasn't shown
and thus not detected.

mercurial/hgweb/server.py

@@ -153,7 +153,8 b' class _httprequesthandler(BaseHTTPServer'
             self.send_header(*h)
             if h[0].lower() == 'content-length':
                 self.length = int(h[1])
-        if self.length is None:
+        if (self.length is None and
+            saved_status[0] != common.HTTP_NOT_MODIFIED):
             self._chunked = (not self.close_connection and
                              self.request_version == "HTTP/1.1")
             if self._chunked:

tests/get-with-headers.py

@@ -31,6 +31,8 b' def request(host, path, show):'
     conn.request("GET", '/' + path, None, headers)
     response = conn.getresponse()
     print response.status, reasons.get(response.reason, response.reason)
+    if show[:1] == ['-']:
+        show = [h for h, v in response.getheaders() if h.lower() not in show]
     for h in [h.lower() for h in show]:
         if response.getheader(h, None) is not None:
             print "%s: %s" % (h, response.getheader(h))

tests/test-hgweb.t

@@ -310,8 +310,10 b' Test the access/error files are opened i'
 
 static file
 
-  $ "$TESTDIR/get-with-headers.py" --twice localhost:$HGPORT 'static/style-gitweb.css'
+  $ "$TESTDIR/get-with-headers.py" --twice localhost:$HGPORT 'static/style-gitweb.css' - date etag server
   200 Script output follows
+  content-length: 4619
+  content-type: text/css
   
   body { font-family: sans-serif; font-size: 12px; margin:0px; border:solid #d9d8d1; border-width:1px; margin:10px; }
   a { color:#0000cc; }