DB connection leak in DAVServlet

When using basic authentication, the DAVServlet leaks a connection each time a session is authenticated.
The authenticate method constructs a Context (which in turn allocates a DB connection), but the Context is not closed or returned from the method in the case where a "WWW-Authenticate" header is sent back.

Also, if an error occurs in the processing of the authentication, the context is also not closed.

I have attached patches for versions 1.5.2, 1.6.2, 1.7.2 and 1.8.0.