| Returns a String identifying the authentication challenge. This is
used, in combination with the host and port to determine if
authorization has already been attempted or not. Schemes which
require multiple requests to complete the authentication should
return a different value for each stage in the request.
Additionally, the ID should take into account any changes to the
authentication challenge and return a different value when appropriate.
For example when the realm changes in basic authentication it should be
considered a different authentication attempt and a different value should
be returned.
This method simply returns the realm for the challenge.
String a String identifying the authentication challenge. Thereturned value may be null. | 