def set(self, key, val):
key_lower = key.lower()<EOL>new_vals = key, val<EOL>vals = self._container.setdefault(key_lower, new_vals)<EOL>if new_vals is not vals:<EOL><INDENT>self._container[key_lower] = [vals[<NUM_LIT:0>], vals[<NUM_LIT:1>], val]<EOL><DEDENT>
Sets a header field with the given value, removing
previous values.

Usage::

    headers = HTTPHeaderDict(foo='bar')
    headers.set('Foo', 'baz')
    headers['foo']
    > 'baz'