Fix FacebookResponse.is_success to properly handle API returns in csv format

[marcus-laia]

The SDK breaks when trying to run an API call that returns a string body (in csv format, for example).

This is what happens:
json_body = self.json()

• it fails to convert the body from string to dict, so it returns the string body itself.

if isinstance(json_body, collections_abc.Mapping) and 'error' in json_body:

• if fails because it checks if json_body is a dict when looking for a dictionary key in it

elif bool(json_body):

• it's basically just checking that json_body is not empty (str or dict) or False.

if 'success' in json_body:

• it doesn't check json_body is a dict
• whenever json_body is a string the code is looking for a substring instead of a dictionary key

return json_body['success']

• if the string body contains 'success', it will throw an exception when trying to use a string to index a string
• I've seen this happening when trying to read ad insights with 'success' in a campaign name

return 'Service Unavailable' not in json_body

• if the string body doesn't contain 'success' but it contains 'Service Unavailable', for any reason, this would return False even if the API call was successful.

[meta-codesync]

@stcheng has imported this pull request. If you are a Meta employee, you can view this in D86377538.