Ability to pass custom headers and parameters in modelSpec

[jameslamine]

I'm using a custom endpoint. Right now I have to make a new endpoint every time I want a modelSpec with different headers or custom parameters.

Could you add the ability to set headers and parameters in modelSpec? If the limitation is that you can only set headers and parameters when you instantiate the client, under the hood it could just make a new API client / endpoint for each model spec.