keepalive: send HTTP request headers in a deterministic order

An upcoming patch will add low-level testing of the bytes being sent
over the wire. As part of developing that test, I discovered that the
order of headers in HTTP requests wasn't deterministic. This patch
makes the order deterministic to make things easier to test.

mercurial/keepalive.py

@@ -298,11 +298,12 b' class KeepAliveHandler(object):'
 
     def _start_transaction(self, h, req):
         # What follows mostly reimplements HTTPConnection.request()
-        # except it adds self.parent.addheaders in the mix.
-        headers = dict(self.parent.addheaders)
-        headers.update(req.headers)
-        headers.update(req.unredirected_hdrs)
-        headers = dict((n.lower(), v) for n, v in headers.items())
+        # except it adds self.parent.addheaders in the mix and sends headers
+        # in a deterministic order (to make testing easier).
+        headers = util.sortdict(self.parent.addheaders)
+        headers.update(sorted(req.headers.items()))
+        headers.update(sorted(req.unredirected_hdrs.items()))
+        headers = util.sortdict((n.lower(), v) for n, v in headers.items())
         skipheaders = {}
         for n in ('host', 'accept-encoding'):
             if n in headers: