Make Candid JS library backward compatible

[chenyan-dfinity]

https://dfinity.atlassian.net/browse/ICSUP-2629

The JS library is not conforming to the spec. Decoding message record { } as type record { a : opt int } results in an error with field a not found. This means that the Candid JS binding has to always use the latest did file if there is an upgrade on the server side.

[krpeacock]

This would be fine if we interpreted the type as { a ?: number }. Any case where we interpret opt as an array will fail this standard.

This relates to dfinity/candid#300