Use directory_must_use_begindir to predict we'll surely use begindir

Previously, we used !directory_fetches_from_authorities() to predict
that we would tunnel connections.  But the rules have changed
somewhat over the course of 0.2.8

changes/bug19191

@@ -0,0 +1,5 @@
+  o Minor bugfixes (downloading):
+    - Predict more correctly whether we'll be downloading over HTTP when we
+      determine the maximum length of a URL. This should avoid a "BUG"
+      warning about the Squid HTTP proxy and its URL limits. Fixes bug 19191;
+      bugfix on ?????.

src/or/routerlist.c

@@ -4674,7 +4674,7 @@ max_dl_per_request(const or_options_t *options, int purpose)
   }
   /* If we're going to tunnel our connections, we can ask for a lot more
    * in a request. */
-  if (!directory_fetches_from_authorities(options)) {
+  if (directory_must_use_begindir(options)) {
     max = 500;
   }
   return max;