[fixes] facebook login fixes for https://github.com/frappe/erpnext/issues/8294 (#3108)

frappe/www/login.py

@@ -43,7 +43,7 @@ def login_via_github(code, state):
 
 @frappe.whitelist(allow_guest=True)
 def login_via_facebook(code, state):
-	login_via_oauth2("facebook", code, state)
+	login_via_oauth2("facebook", code, state, decoder=json.loads)
 
 @frappe.whitelist(allow_guest=True)
 def login_via_frappe(code, state):