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.

# Utility for replacing the simple input field for the timezone with
# a select-field that lists the available values.

import cgi

try:
    import pytz
except ImportError:
    pytz = None


def tzfield(prop, name, default):
    if pytz:
        value = prop.plain()        
        if '' == value:
            value = default
        else:
            try:
                value = "Etc/GMT%+d" % int(value)
            except ValueError:
                pass

        l = ['<select name="%s">' % name]
        for zone in pytz.all_timezones:
            s = ' '
            if zone == value:
                s = 'selected=selected '
            z = cgi.escape(zone)
            l.append('<option %svalue="%s">%s</option>' % (s, z, z))
        l.append('</select>')
        return '\n'.join(l)
        
    else:
        return prop.field()

def init(instance):
    instance.registerUtil('tzfield', tzfield)