plugins/api: use headers.set(), not headers.update()

plugins/api: use headers.set(), not headers.update()

The werkzeug.Response().headers do not offer an update() method as
the same key can be twice in the header 'dict'. Thus, iterate over
the header keys and use header.set(key, value) which replaces an
existing header key.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>