Don't support anything other than Basic auth.

We should disregard authentication schemes other than
"Basic" and let clients handle them themselves.

The java Authenticator API gives us a user name and
password combination, but we can't know how to format
that information for any scheme other than basic.

Historically:

The JB implementation responds to challenges from an
arbitrary scheme "X" by sending a header with scheme
"X" but formatted like the "Basic" scheme.

The current implementation responds to challenges from
an arbitrary scheme "X" by sending a header with scheme
"Basic" and formatter like the "Basic scheme".

Partial fix for test cases in URLConnectionTest:
- testAuthenticateWithCommaSeparatedAuthenticationMethods
- testAuthenticateWithMultipleAuthenticationHeaders

Change-Id: I110844dd104b7673f987ccb89e7a63db2b4e50a3