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