[FIX] pad: Use a post instead of a get on the request

The old code used Request(data=)]( https://github.com/odoo-dev/odoo/blob/723b083cf5e51beec166059350466e5504db46c9/addons/pad/py_etherpad/__init__.py#L39)

Which however means it did a POST:

> the HTTP request will be a POST instead of a GET when the data parameter is provided

yet the code currently uses `requests.get`, so [it looks very much like a mistake when converting from urllib to requests](https://github.com/odoo-dev/odoo/commit/01e35141479b8dfc4a390518f89fc52e7cec5396#diff-3303eb022ac36e1ef7258c28f24f6de6).

addons/pad/py_etherpad/__init__.py

@@ -30,7 +30,7 @@ class EtherpadLiteClient:
         params = arguments or {}
         params['apikey'] = self.apiKey
 
-        r = requests.get(url, data=params, timeout=self.TIMEOUT)
+        r = requests.post(url, data=params, timeout=self.TIMEOUT)
         r.raise_for_status()
         return self.handleResult(r.json())