'Set or append a header for this response.
Warning:
If the header already exists, the new value will be appended
to it, delimited by a comma. Most header specifications support
this format, Set-Cookie being the notable exceptions.
Warning:
For setting cookies, see :py:meth:`~.set_cookie`
Args:
name (str): Header name (case-insensitive). The restrictions
noted below for the header\'s value also apply here.
value (str): Value for the header. Must be of type ``str`` or
``StringType`` and contain only US-ASCII characters.
Under Python 2.x, the ``unicode`` type is also accepted,
although such strings are also limited to US-ASCII.'

def append_header(self, name, value):

    if PY2:
        name = str(name)
        value = str(value)
    name = name.lower()
    if (name in self._headers):
        value = ((self._headers[name] + ',') + value)
    self._headers[name] = value
