mod_auth_form: Make sure the input filter stack is properly set

up before reading the login form. Make sure the kept body filter
is correctly inserted to ensure the body can be read a second
time safely should the authn be successful. [Graham Leggett,
Ruediger Pluem]


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@654958 13f79535-47bb-0310-9956-ffa450edef68