Typespec mismatch with HTTPoison.process_response/1

[fastjames]

Running mix dialyzer returns a number of errors in Forcex.Api.Http and Forcex.Bulk around the override functions for process_response/1. HTTPoison changed the signature for that function several years ago, such that headers has been changed from a Map to a kwlist. This makes the function guards (used in both locations) no longer feasible.

These functions should be updated to work with headers as a keyword list.

[fastjames]

Since dialyzer has a couple of distinct errors, I wanted to document them individually here. I think I have code that's sufficient to fix this error, so I can either extract it from my current branch as its own PR or include it in a big ball of fixups.