None should be translated to "none" where appropriate

[aaronsegura]

Nextcloud expects the string "none" to be sent in some API calls. Using the python value of None causes errors. These values should be translated automatically to preserve pythony pythonness.

[JocelynDelalande]

Same for

• True -> 'true'
• False -> 'false'

In current code base, get_file_shares() asks to provide Python bools for reshares and subfiles args. But they translate to 'True' and 'False' strings, which are not recognized by NC API. So current workaround is to provide 'true' or 'false' as arg values.

[JocelynDelalande]

For boolean, it's fixed a non generic way via #26
(I forgot to update, oops !)