rgw: fix use of libcurl with empty header values

Fixes: http://tracker.ceph.com/issues/23663

Signed-off-by: Casey Bodley <cbodley@redhat.com>

src/rgw/rgw_http_client.cc

@@ -352,8 +352,13 @@ static curl_slist *headers_to_slist(param_vec_t& headers)
 
     val = camelcase_dash_http_attr(val);
 
-    val.append(": ");
-    val.append(p.second);
+    // curl won't send headers with empty values unless it ends with a ; instead
+    if (p.second.empty()) {
+      val.append(1, ';');
+    } else {
+      val.append(": ");
+      val.append(p.second);
+    }
     h = curl_slist_append(h, val.c_str());
   }