Fix bug preventing update from overwriting existing data

[iffies]

According to its documentation, Service.update/3 uses a PUT request when :overwrite is set to true in its Keyword list of options. This will "overwrite all values in the destination record with values being sent".

But because Keyword.fetch/2 is used instead of Keyword.fetch!/2, :overwrite will never pattern-match on true, only {:ok, true}. So it is always interpreted as false, forcing a PATCH request.

Given that Keyword.fetch!/2 is used throughout the code with its risk of raising errors, I first assumed it was intended here. But best to use safer code in second commit, which I think was the actual intent here.