Adds spec for #reset
Adds spec for #reset

* Previously there was no test for update_speller
being run as part of a key reset:
```
+----------+---------------------------+-------+--------+-------------------+
| coverage | file                      | lines | missed | missing           |
+----------+---------------------------+-------+--------+-------------------+
|  96.33%  | lib/ffi/aspell/speller.rb | 109   | 4      | 141-142, 302, 369 |
+----------+---------------------------+-------+--------+-------------------+
``` 
* This adds a check to reset a key so that 
`update_speller` gets called, and line `369` is
now covered! :dancer:
YorickPeterse/ffi-aspell