free `response` after every get* function

Hi @dinhviethoa
I found a memory leak in `response`.

Here is your implementation in `libetpan`: https://github.com/dinhviethoa/libetpan/blob/4ccec06771f0f8d518a407664b49228a5dc194a7/src/low-level/imap/mailimap.c#L662

I found u call `mailimap_response_free` to free up this object, then shall we do the same to fix the memory leak?