Bugfix - allow custom input variables in chat zero shot agent's prompt (#1624)
I was trying out the `chat-zero-shot-react-description` agent for
[qabot](https://github.com/hardbyte/qabot/blob/dbbd31bb2702d433ed573bb177f1d1272d36f2ae/qabot/agents/data_query_chain.py#L35-L52)
but langchain 0.0.108 doesn't correctly use custom 'input_variables` in
the prompt template.