correction for the limit option

Many commmands which use the app_list fonction not worked fine.

to reproduce : 
* sudo yunohost app install ttrss_ynh
* sudo yunohost app -f ttrss -l 1
the second command will not return the good response.

consequence, we can't uninstall ttrss for example.

the call trace : 
```python
Traceback (most recent call last):
  File "test.py", line 3, in <module>
    print app_list(filter='ttrss_ynh', raw=True, limit=1)['ttrss_ynh']
KeyError: 'ttrss_ynh'
```