add kwargs to authenticate method
add kwargs to authenticate method

This way, when using multiple authentication backends (ie, a custom django auth backend and custom Saml2Backend) and the first custom backend does not authenticate the user, this other backend can process the next authentication request attempt without crashing.
knaperek/djangosaml2,BetterWorks/djangosaml2,writepython/djangosaml2,WebSpider/djangosaml2,BetterWorks/djangosaml2,writepython/djangosaml2,knaperek/djangosaml2,WebSpider/djangosaml2