CAS-1411: Update protocol spec with details on proxy callback behavior

The commit updates the protocol to provide more implementation detail as for the conditions that need to be passed and verified for a successful proxy authentication scenario. Emphasis is on failure scenarios. Furthermore, changed the recommendation on the format of attributes that are extended in Appendix A to match the default behavior.

Relevant sections from the updated protocol:

>CAS uses an HTTP GET request to pass the HTTP request parameters "pgtId" and
    "pgtIou" to the pgtUrl endpoint. These entities are discussed in Sections [3.3](#head3.3) and
    [3.4](#head3.4), respectively. If the proxy callback url specifies any parameters, those
    MUST be preserved. CAS MUST also ensure that the endpoint is reachable by verifying 
    the response HTTP status code from the GET request, as detailed in step #3. If the
    proxy service fails to authenticate or the endpoint responds with an unacceptable status
    code, proxy authentication MUST fail and CAS MUST respond with the appropriate error code
    as is described in section [2.5.3](<#head2.5.3>).