Add tests (#171)

* test models listing

* remove non-needed method

* test for .streamFinished

* add more error tests

* improve stream testing

* fix typo

chat_stream.go

@@ -78,10 +78,6 @@ func (stream *ChatCompletionStream) Close() {
 	stream.response.Body.Close()
 }
 
-func (stream *ChatCompletionStream) GetResponse() *http.Response {
-	return stream.response
-}
-
 // CreateChatCompletionStream — API call to create a chat completion w/ streaming
 // support. It sets whether to stream back partial progress. If set, tokens will be
 // sent as data-only server-sent events as they become available, with the