[fix] self_info: request.user_agent is not a str

The user_agent attribute of the Flask request object is an instance of
the werkzeug.user_agent.UserAgent class.

This will fix the following error of the self_info plugin:

> ERROR:searx.plugins.self_info: Exception while calling post_search
> Traceback (most recent call last):
>   File "searx/plugins/__init__.py", line 203, in call
>     ret = getattr(plugin, plugin_type)(*args, **kwargs)
>   File "searx/plugins/self_info.py", line 31, in post_search
>     search.result_container.answers['user-agent'] = {'answer': gettext('Your user-agent is: ') + ua}
> TypeError: can only concatenate str (not "UserAgent") to str

0 files changed, 0 insertions, 0 deletions