fix(web): issue2551356. Add etag header for not-modified (304) request. When a 304 is returned to a conditional request for a static file, print an ETag for the response. ETag was always sent with a 200 response. This also adds initial support for if-none-match conditional requests for static files. Changes: Refactors the if-modified-since code out to a method. It moves a file stat call from serve_static_file to _serve_file so that an etag can be generated by both serve_static_file and serve_file which call _serve_file. Tests added. This does not test the codepath where serve_file pulls content from the database rather than from a local file on disk. Test mocking _serve_file changed to account for 5th argument to serve_file BREAKING CHANGE: function signature for client.py-Client::_serve_file() now has 5 not 4 parameters (added etag param). Since this is a "hidden" method I am not too worried about it.

#  - "path/to/folder"  # ignore folders and all its contents
#  - "test_*.rb"       # wildcards accepted
#  - "**/*.py"         # glob accepted
#  - "[a-z]+/test_.*"  # regexp accepted

# ignore files that are build utils and not executed
# code, or are deprecated.
ignore:
   - "roundup/cgi/TAL/talgettext.py" # utility command
   - "roundup/cgi/TAL/DummyEngine.py" # test harness
   - "roundup/cgi/apache.py"  # mod_python is deprecated
   - "roundup/install_util.py" # another build utlity
   - "roundup/dist"           # more build utils
   - "roundup/test"           # code used for testing