Default implementation of
UserTokenHandler. This class will use
an instance of
Principal as a state object for HTTP connections,
if it can be obtained from the given execution context. This helps ensure
persistent connections created with a particular user identity within
a particular security context can be reused by the same user only.
DefaultUserTokenHandler will use the user principal of connection
based authentication schemes such as NTLM or that of the SSL session
with the client authentication turned on. If both are unavailable,
null token will be returned.