Fix EncOption/DecOption unset fields on mode regurgitation.

[benluddy]

Description

I noticed that DefaultMapType didn't survive the trip from DecOptions to DecMode and back. There are existing roundtrip tests for both DecOptions/DecMode and EncOptions/EncMode, but they also depend on being updated when new options are added. I've updated the existing tests so that they fail unless all fields have been set to a non-zero value, and set the fields that were missing in the implementation of EncOptions() and DecOptions(). Hopefully the updated tests will make it easy to avoid the same mistake in the future.

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[fxamacker]

LGTM! 👍 Thank you for catching this bug, fixing it, and improving tests!

[fxamacker]

Thank you for checking for zero value option programmatically! 👍

[fxamacker]

Thank you for adding this detailed comment! 👍