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.

@import url(style.css);

body { padding: 0;}
body > .header,
body > .footer
{
  margin: 0;
}
body .content { width: auto;}
.header h1 { font-size: 110%; }

table.classhelp th
{
  border-top: 1px solid #afafaf;
  border-bottom: 1px solid #afafaf;
  empty-cells: show;
  text-align: left;
  vertical-align: middle;
  white-space: nowrap;
}

table.classhelp td
{
  border-bottom: 1px solid #efefef;
  empty-cells: show;
  text-align: left;
  vertical-align: middle;
  white-space: nowrap;
}