bundle2: fix assertion that 'compression' hasn't been set stable

bundle2: fix assertion that 'compression' hasn't been set

`n.lower()` will return `compression`, not `Compression`.

--- a/mercurial/bundle2.py	Fri Feb 10 18:20:58 2017 +0100
+++ b/mercurial/bundle2.py	Mon Feb 13 11:43:12 2017 -0800
@@ -518,7 +518,7 @@
         """setup core part compression to <alg>"""
         if alg in (None, 'UN'):
             return
-        assert not any(n.lower() == 'Compression' for n, v in self._params)
+        assert not any(n.lower() == 'compression' for n, v in self._params)
         self.addparam('Compression', alg)
         self._compengine = util.compengines.forbundletype(alg)
         self._compopts = compopts