fix: map record decodes in union case

[nrwiersma]

This fixes a potential bug where map record decodes where optimised changing how unions are decoded.

See aa7f619#diff-76d79b5f35aac6861a49a410fbb009f12801faadce2db0f35d21f4c50bfe3796R279

Fixes #386

[nrwiersma]

@redaLaanait If you have a moment can you check this out please.

[redaLaanait]

This fix LGTM:

It seems that EfaceDecoder should not be used directly In the case of Union with a receiver-of-type interface. This condition was added to the decoderOfType factory here

The optimization change you pointed out does omit this check as it uses the newEfaceDecoder instead of passing through the factory.