cptools.SessionAuth: fix login_screen page handler

The login_screen page handler erroneously returned the formatted HTML page,
which was simply interpreted as a boolean that controls whether the call
to the controller function was suppressed or not.

Set the response.body instead and return a literal True that suppresses the
call to the page handler.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>

--HG--
branch : fix-session-auth-login-screen
Merged in bazsi/cherrypy/fix-session-auth-login-screen (pull request #60)

fix an issue that causes @session_auth tool to fail if login_screen is requested

--HG--
branch : cherrypy-3.2.x
Merge with cherrypy-3.2.x
Extract variable for nicer indentation
Extract message to perform debug messages

--HG--
extra : amend_source : 11279881e80a3cd6c856efe75b88936f71e949e2