hgweb: ensure Content-Length and Content-Type are not promoted to HTTP_ on py3

In stabilizing test-lfs-serve-access.t for py3, the server started asserting on
blob upload:

    Environment should not have the key: HTTP_CONTENT_LENGTH (use CONTENT_LENGTH instead)

It could be avoided by explicitly setting the Content-Length header on the
client side.  I didn't go back to py2, but printing the original header here in
py37 revealed 'Content-length' when sent to the error log.

mercurial/hgweb/server.py

@@ -165,7 +165,7 b' class _httprequesthandler(httpservermod.'
         if length:
             env[r'CONTENT_LENGTH'] = length
         for header in [h for h in self.headers.keys()
-                       if h not in (r'content-type', r'content-length')]:
+                      if h.lower() not in (r'content-type', r'content-length')]:
             hkey = r'HTTP_' + header.replace(r'-', r'_').upper()
             hval = self.headers.get(header)
             hval = hval.replace(r'\n', r'').strip()