Only send the if_modified_since header for a v3 consensus.

Spotted by xmux; bugfix on 0.2.0.10-alpha.
(Bug introduced by 20b10859)

ChangeLog

@@ -1,3 +1,11 @@
+Changes in Version 0.2.1.21 - 20??-??-??
+  o Minor bugfixes:
+    - Do not refuse to learn about authority certs and v2 networkstatus
+      documents that are older than the latest consensus.  This bug might
+      have degraded client bootstrapping.  Bugfix on 0.2.0.10-alpha.
+      Spotted and fixed by xmux.
+
+
 Changes in version 0.2.1.20 - 2009-10-15
   o Major bugfixes:
     - Send circuit or stream sendme cells when our window has decreased

src/or/directory.c

@@ -332,7 +332,7 @@ directory_get_from_dirserver(uint8_t dir_purpose, uint8_t router_purpose,
       return;
   }
 
-  if (DIR_PURPOSE_FETCH_CONSENSUS) {
+  if (dir_purpose == DIR_PURPOSE_FETCH_CONSENSUS) {
     networkstatus_t *v = networkstatus_get_latest_consensus();
     if (v)
       if_modified_since = v->valid_after + 180;