MINOR: WURFL: call header_retireve_callback() in dummy library

The current coverage of the dummy library was limited because the callbacks
passed to wurfl_lookup() were not called. Now we do call them with one existing
and one non-existing headers to make sure that ha_wurfl_retrieve_header() is
covered by the tests as well.

contrib/wurfl/dummy-wurfl.c

@@ -67,6 +67,11 @@ wurfl_error wurfl_load(wurfl_handle hwurfl)
 
 wurfl_device_handle wurfl_lookup(wurfl_handle hwurfl, wurfl_header_retrieve_callback header_retrieve_callback, const void *header_retrieve_callback_data)
 {
+  // call callback, on a probably existing header
+  const char *hvalue = header_retrieve_callback("User-Agent", header_retrieve_callback_data);
+  // and on a non existing one
+  hvalue = header_retrieve_callback("Non-Existing-Header", header_retrieve_callback_data);
+
   return (void *) 0xdeffa;
 }