The `HTTPClient` class was missing the `export` method.

Add it in order for the gspread `Client` and the
`Spreadsheet` class to use it to export spreadsheets.

closes #1385

Signed-off-by: Alexandre Lavigne <[email protected]>

Add new test that exports a spreadsheet, using
the gspread client and the spreadsheet itself.

We serialize our requests into a JSON file.
JSON cannot serialize binary data (like PDF)
so we can only test with CSV.

Signed-off-by: Alexandre Lavigne <[email protected]>

Add missing methods to the HTTP client.

update code so the gspread Client and the spreadsheet use
these new methods in order to:
- insert
- list
- remove

a permission.

We can't test these function automatically as this requires
exposing personal information in the recorded requests.

We'll add more tests later when we find a way.

closes #1385

Signed-off-by: Alexandre Lavigne <[email protected]>

Signed-off-by: Alexandre Lavigne <[email protected]>