digest_sspi: Fix nonce-count generation in HTTP digest

- on the first invocation: keep security context returned by
  InitializeSecurityContext()

- on subsequent invocations: use MakeSignature() instead of
  InitializeSecurityContext() to generate HTTP digest response

Bug: https://github.com/curl/curl/issues/870
Reported-by: Andreas Roth

Closes https://github.com/curl/curl/pull/1251

lib/urldata.h

@@ -408,6 +408,7 @@ struct digestdata {
 #if defined(USE_WINDOWS_SSPI)
   BYTE *input_token;
   size_t input_token_len;
+  CtxtHandle *http_context;
 #else
   char *nonce;
   char *cnonce;