@fluent<EOL><INDENT>def header(self, key, value):<DEDENT>
if type(key) is tuple:<EOL><INDENT>key, value = str(key[<NUM_LIT:0>]), key[<NUM_LIT:1>]<EOL><DEDENT>headers = {key: value}<EOL>self._headers.extend(headers)<EOL>
Defines a new response header.
Alias to ``Response.header()``.

Arguments:
    header (str): header name.
    value (str): header value.

Returns:
    self: ``pook.Response`` current instance.