Manage record fields with default values when reading from binary

[faberia]

This handles the case of a compatible schema evolution, when new fields with default values are added.
With this, consumers using the new schema can read messages produced with the old schema.

[karrick]

This is a really great idea!

It seems one disadvantage of this is that it only works when the entire buffer is empty. So it would only work if it were the last outer-most record (if records embedded in records) in a byte stream.

Which makes me concerned that the default handling for binary records would be different whether or not extra bytes were available to read.

Can we talk about this a bit more? I'd love to see this feature integrated, and clearly the implementation is done well. I am just thinking about the other cases.

[faberia]

Hi @karrick,
Sure, let's make this feature as good as we can before merging it. Do you know how we could know we are in one of the cases you refer to?
Also, I found out that the default value is not always set with the right type (eg: float64 instead of int). I added another commit with a possible fix.