Skip discriminator entries when decoding inline-map sealed children

[tadfisher]

Fixes #3172

Streaming JSON decoding is missing an edge-case where:

• an object is decoded in MAP mode
• the object contains a polymorphic discriminator

I believe the only case this can happen is when you have a value class, wrapping Map<K, V>, in a sealed hierarchy. If we're in this case, skip the discriminator entry and proceed to decode the object as a Map.

Tree decoding doesn't handle the discriminator either; skip it by filtering it out of keys.

There's an ancillary issue the test revealed: encoding the value class with JsonTreeEncoder is missing the discriminator entirely, probably because we don't set a tag for Map values. Added similar handling for the case where we have a discriminator but no tag, and return this decoder so that we encode the wrapped value without dealing with tags.