Send Authorization header irrespective of previous authentication sta…

…tus (requests#178)

- Currently 'force_preemptive' flag sends the 'Authorization' header only for the
  first call and not for subsequent calls. This is because 'Authorization' header
  is preemptively sent only if the request is not authenticated. The status of
  authentication is stored as 'auth_done' instance variable.
  Since all the HTTP calls use the same instance of HTTPKerberosAuth class
  the 'auth_done' instance variable is set to True after first successful
  authentication and thus 'Authorization' header is not set preemptively
  after that.
- The fix is to send the Authorization header irrespective of the
  authentication status of previous call.

requests_kerberos/kerberos_.py

@@ -427,7 +427,7 @@ def deregister(self, response):
         response.request.deregister_hook('response', self.handle_response)
 
     def __call__(self, request):
-        if self.force_preemptive and not self.auth_done:
+        if self.force_preemptive:
             # add Authorization header before we receive a 401
             # by the 401 handler
             host = urlparse(request.url).hostname