hgweb: allow Content-Security-Policy header on 304 responses (issue5844) stable

hgweb: allow Content-Security-Policy header on 304 responses (issue5844)

A side-effect of 98baf8dea553 was that the Content-Security-Policy
header was set on all HTTP responses by default. This header wasn't
in our list of allowed headers for HTTP 304 responses. This would
trigger a ProgrammingError when a 304 response was issued via hgwebdir.

This commit adds Content-Security-Policy to the allow list of headers
for 304 responses so we no longer encounter the error.

Differential Revision: https://phab.mercurial-scm.org/D3436

--- a/mercurial/hgweb/request.py	Mon Apr 30 17:22:20 2018 -0700
+++ b/mercurial/hgweb/request.py	Mon Apr 30 17:28:59 2018 -0700
@@ -473,6 +473,7 @@
                           if k.lower() not in ('date', 'etag', 'expires',
                                                'cache-control',
                                                'content-location',
+                                               'content-security-policy',
                                                'vary')}
             if badheaders:
                 raise error.ProgrammingError(

--- a/tests/test-hgweb-csp.t	Mon Apr 30 17:22:20 2018 -0700
+++ b/tests/test-hgweb-csp.t	Mon Apr 30 17:28:59 2018 -0700
@@ -57,8 +57,8 @@
   $ get-with-headers.py --twice --headeronly localhost:$HGPORT repo1/static/style.css content-security-policy
   200 Script output follows
   content-security-policy: script-src https://example.com/ 'unsafe-inline'
-  500 Internal Server Error
-  [1]
+  304 Not Modified
+  content-security-policy: script-src https://example.com/ 'unsafe-inline'
 
 repo page should send CSP by default, include etag w/o nonce