'Create a new response header, replacing any previously defined
headers with the same name. This equals ``response[name] = value``.:param append:
Do not delete previously defined headers. This can
result in two (or more) headers having the same name.'

def set_header(self, name, value, append=False):

    if append:
        self._headers.setdefault(_hkey(name), []).append(str(value))
    else:
        self._headers[_hkey(name)] = [str(value)]
