- Fix pass system_message argument in new feature openai_functions_agent (#6297)

can't pass system_message argument, the prompt always show default
message "System: You are a helpful AI assistant."
```
system_message = SystemMessage(
    content="You are an AI that provides information to Human regarding documentation."
)
agent = initialize_agent(
    tools,
    llm=openai_llm_chat,
    agent=AgentType.OPENAI_FUNCTIONS,
    system_message=system_message,
    agent_kwargs={
        "system_message": system_message,
    },
    verbose=False,
)
```

#### Who can review?

Tag maintainers/contributors who might be interested:

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  - @agola11

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  - @eyurtsev

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langchain/agents/openai_functions_agent/base.py

@@ -236,12 +236,15 @@ class OpenAIFunctionsAgent(BaseSingleActionAgent):
         llm: BaseLanguageModel,
         tools: Sequence[BaseTool],
         callback_manager: Optional[BaseCallbackManager] = None,
+        system_message: Optional[SystemMessage] = SystemMessage(
+            content="You are a helpful AI assistant."
+        ),
         **kwargs: Any,
     ) -> BaseSingleActionAgent:
         """Construct an agent from an LLM and tools."""
         if not isinstance(llm, ChatOpenAI):
             raise ValueError("Only supported with OpenAI models.")
-        prompt = cls.create_prompt()
+        prompt = cls.create_prompt(system_message=system_message)
         return cls(
             llm=llm,
             prompt=prompt,