Fix an internal test failure (google-gemini#279)

Picked this up when adding missing tests on the internal copy. I think
the failure is spurious but this fix is easy enough ¯\_(ツ)_/¯

Author: markmcd

google/generativeai/notebook/text_model.py

@@ -41,7 +41,8 @@ def _generate_text(
 
         model_name = model or _DEFAULT_MODEL
         gen_model = genai.GenerativeModel(model_name=model_name)
-        return gen_model.generate_content(prompt, generation_config=gen_config)
+        gc = genai.types.generation_types.GenerationConfig(**gen_config)
+        return gen_model.generate_content(prompt, generation_config=gc)
 
     def call_model(
         self,