hgweb: discard Content-Type header for 304 responses (issue5844)

A side-effect of 98baf8dea553 was that hgwebdir always sets a global
default for the Content-Type header. HTTP 304 responses don't allow
the Content-Type header. So a side-effect of this change was that
HTTP 304 responses served via hgwebdir resulted in a ProgrammingError
being raised.

This commit teaches our 304 response issuing code to drop the
Content-Type header.

Differential Revision: https://phab.mercurial-scm.org/D3435

mercurial/hgweb/hgweb_mod.py

@@ -399,6 +399,12 b' class hgweb(object):'
                 tag = 'W/"%d"' % self.mtime
                 if req.headers.get('If-None-Match') == tag:
                     res.status = '304 Not Modified'
+                    # Content-Type may be defined globally. It isn't valid on a
+                    # 304, so discard it.
+                    try:
+                        del res.headers[b'Content-Type']
+                    except KeyError:
+                        pass
                     # Response body not allowed on 304.
                     res.setbodybytes('')
                     return res.sendresponse()

tests/test-hgweb.t

@@ -890,9 +890,7 b' HTTP 304 works with hgwebdir (issue5844)'
   200 Script output follows
   content-length: 2677
   content-type: text/css
-  500 Internal Server Error
-  transfer-encoding: chunked
-  [1]
+  304 Not Modified
 
   $ killdaemons.py