[CORE-5238] Use string for database endpoint params

Followup to #8. JSON encoding the params resulted in URL encoded JSON
object in query string sent to the Metabase API.

The requests package expects a dictionary of strings for `params` [1]
with no rules to encode boolean types [2, 3], so best to keep it simple
and use strings.

[1]: https://docs.python-requests.org/en/latest/user/quickstart/#passing-parameters-in-urls
[2]: psf/requests#2999
[3]: psf/requests#3355

intake_metabase/source.py

@@ -214,7 +214,7 @@ def get_databases(self):
         self._create_or_refresh_token()
 
         headers = _merge_dicts({'X-Metabase-Session': self._token}, self.extra_headers)
-        params = json.dumps({'include': 'tables', 'saved': True})
+        params = {'include': 'tables', 'saved': 'true'}
 
         res = requests.get(
             urljoin(self.domain, '/api/database'),