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This Handler option is used it to allow/deny request by their method.

The API is an RPC API so it normally should only work with PUT, but we also
use a read-only, GET-based partial API that runs with the gateway.

This commit makes the actual allowed (or handled) methods configurable.

A test is added, and test facilities improved to be able to set/modify
this option.

This is a breaking change and will affect Filecoin. However, filecoin is
currently using this incorrectly anyways (breaking PreRun and PostRun).

- always return command errors in Next, neven in Send
- also check trailer when receiving EOF
- update tests to reflect changes

These aren't placeholder contexts.