Added "public" to Cache-Control header.

"max-age" already indicates that it can be cached, but it can't hurt
to allow it explicitly.

src/libravatarserv.cpp

@@ -31,7 +31,7 @@ settings::Settings settings::settings;
 int main()
 {
     cout << "Server: libravatarserv/" << global::version << endl;
-    cout << "Cache-Control: max-age=86400\n";
+    cout << "Cache-Control: public, max-age=86400\n";
     cout << "Connection: close\n";
 
     const char *request = std::getenv("REQUEST_URI");